Also, it's quite impossible to understand why one opening line yields a += and another just = if you don't know many "deep" middlegame concepts to begin with. It's impossible to begin by picking up an advanced opening book, nothing will make sense. This is mostly why opening study (beyond traps or known bad lines) isn't very useful even up to 1800 USCF IMO and possibly as high as 2000.
A game may hinge on a single tactic, but most moves in a game are not tactical moves, and to reach a position where the tactics are favoring you, you have to know other things. Tactics are very important, but other knowledge is essential as well.
If your rating is 860 you need to study tactics, then study tactics, then study tactics, then pick up and endgame book for basic mates and some very basic endgame ideas, and then study tactics some more.
At the level you are at, and for quite some time to come, your games are going to swing on tactics. The winner is the one who makes the fewest tactical mistakes, and who gets ahead in material. It will rarely if ever be more subtle than that. Later, yes, there are a huge range of factors to consider, but untill you see all the possible checks and caputures from both side, can calculate a few ply deep, and can spot tactical motifs like pins, forks, skewers, etc, you will get your brains beat out by someone who can. Step one is to be able to keep your peices safe and get a few of the other guy's pieces along the way.
Trying to do deep opening study at your level (or my level for that matter, and im about 500 points ahead of you) is a waste of time. If you are playing turn based chess you can consult opening materials (books, databases, etc) and if you are playing OTB or live online you will find that in your range (a) the other guy doesn't know any theory either, and (b) you will be fine if you play by good opening principles and watch for tactical traps. So, play to control the center, develop your pieces quickly, get the king to safety, etc. But don't spend a bunch of time trying to learn master lines to a deep level. The small advantages that masters are fighting for in the opening are just that, small, and the position will likely be turned on its head the first time someone makes a tactical blunder. Your mission at this point is to make sure you are not that somemone, at least as often as possible.