I think at your level there is going to be a lot of this my blah blah for his blah blah.
As you get better, you will be faster, more thorough and deeper.
One thing that can help is once you have determined if a line can be easily refuted, move on. And try to reduce the number of times you calculate any individual line.
Those will always be the most difficult. You will get better at calculating those combinations as you practice it.
i agree with bmartz ,practice makes perfect :)
Positional attacks are usually those you try and attain an advantage through ane exchange sac... or you make up with by eliminating pawns. For instance, rook for a knight should be compensated with pawns unless you see a winning combination or strong positional attack.
The more you practice it (i.e. the more you play in general) the easier this becomes. Like most skills in chess, it's comforting to know the players around your rating struggle with the same things.
A few tricks until it becomes easier.
1. In a sequence of captures, if a player makes the first and last capture then they've captured more pieces. So as you start to calculate you may say to yourself "white captures first" and then if the last capture is also white you know white captured a greater number.
Pro: easy to do
Cons: Can be confusing if captures aren't in succession. And isn't useful if many different point values are being captured E.g. you won two knights and a pawn but that wasn't worth losing your one queen!
2. After looking at a line, calculate the sequence again, but only count for what white wins e.g. knights=3 rooks =5 and so on. Then calculate it once more and add it up for black and compare the values.
Pro: Captures not in succession isn't a problem. Able to compare exact point value of whole trade.
Con: May require a lot of re-calculating to double check a single line.
3. Keep a running point total as you calculate. Positive numbers for white, negative for black. Each time a capture is made, update the number.
Pro: Not as much calculation as trick #2.
Con: Stresses your working memory while calculating (you'd rather use it for visualization).
Bonus tip: It's not illegal to use your fingers to help with any of these 
Positive numbers are finger on your right hand, negative shift numbers towards left hand.
A good future goal, by the way, is to evaluate a series of trades by accurately imagining the future position of the pieces on the board, not the ones that come off. For example instead of thinking "I gave up a rook for a knight and a bishop" you'll imagine the future position and see that what's left on the board is your two bishops vs his rook (which is much more telling).
And look for your opponents best reply to each of your moves, not just the move you would like him to make.
Sometimes, though, these are 1 and the same, baby.
Thanks so much for all the helpful comments, particularly liked the idea of visualising the board after an exchange combination, will work towards this.Its good to know that it is common problem at this level.
Thanks again Shaun
One simple thing to do is to count the number of times a piece is attacked and the number of times it is defended. If the number is the same exchanges will work out equally, if the piece is defended one less time than attacked then the attacker will gain material.
As better you get as faster in calculating you become. I am only rated 2012 Fide and I often dont need to calculate long because I somehow see if something is working or not. When you have a better understanding of chess, you have a better overview about the board and you see where you lose material and where not. Might Sound crazy but you really get this feeling (Intuition) when you get better at chess and TACTICS.
This method isnt good because its so easy to fail. He will misscalculate or forget one addition. When this happens he loses Material without even knowing why. He will just count his points but have no plan of the lines he analyses.
Okay micky thats fine but what I meant was its not good for him because you need some experience to know what you do
At the end of the day you just need to buckle down and calculate in some positions. Lazy play will show in your results. Looking at tactical puzzles and burning those patterns into your head will save you some time in actual games, but I find this boring. Honestly, the exchange itself is not the hard thing to calculate, but in being able to evaluate the hypothetical positon at the end of the variations and determine which is most favorable to you.
And I don't like the suggestion of subtracting number values as you go about the variation. A Knight and a Bishop are usually stronger than a Rook and a Pawn, yet you wouldn't know this just looking at material as a number, and also you aren't taking into consideration the quality and activity of pieces doing this. Are you taking a blockaded Bishop? It's actually worth 1 point, because it's just a taller Pawn. Bobby Fischer once traded a Rook for a monster Knight (which was not threatening mate or anything) just simply because that Knight's placement made it the strongest piece on the board--with no compensation.
Hi
I'm n ot sure how common this problem is for beginners, I often avoid multi move exchange combinations because have difficulty calculating if It gains or loses material and by how much.( I tend to think like this, if I take his Knight, he takes my bishop, I take... which obviously is confusing and time consuming sure there must be an easier way) Hope you can help
Best Wishes Shaun
Step 1. Acquire thru practice and study the 5 strong visualization pattern memory banks.
Step 2. Learn thoroughly from both the White and Black side the 6 characteristic pawn structures/formations (this includes indicated levers and pawn breaks) that result from almost any opening.
Step 3. Know what opening and variation you are in on the board.
Step 4. A move will seem to jump up off the board and smack you on the forehead in a flash!!
Step 5. Analyze based on that move and select those variations that KILL COUNTERPLAY on the part of your opponent.
Step 6. Look for a better move which is a process that usually dovetails into the variations of the move that seemingly jumped up off the board and smacked you on the forehead.
Steps 5 and 6 give you at least 2 or more different perspectives on exchange calculations. However, the priority in calculating the value of exchange combinations is KILL COUNTERPLAY.
As a consequence of doing all of the above you will have developed a diagram in your mind that replaces any need for a chessboard and pieces for analyzing diagram position on a screen or physical chessboard or even reading chess books.
If you would like to know more about all of this especially how to recognize whatKILLS COUNTERPLAYplease let me know.
Hi
I'm n ot sure how common this problem is for beginners, I often avoid multi move exchange combinations because have difficulty calculating if It gains or loses material and by how much.( I tend to think like this, if I take his Knight, he takes my bishop, I take... which obviously is confusing and time consuming sure there must be an easier way) Hope you can help
Best Wishes Shaun