My aproach goes as follow I started with 400-500, I do 3 tactics if they are all good I raise the level to 450-550, if I make one mistake I keep the level, if I make 2 or 3 mistakes I lower the level with 50. This I do untill I keep stuck and then I restart at the basic and do some other things and try to get higher the next time.
Personally I think the 600 level tactics are a little bit too easy, also for you seeing your turn based rating. However it's important to repeat them sometimes and to make sure that all easy mistakes are eliminated from your games.
Another method which I use in between is leaving the tactic level empty (or make it 600-2000 orso) for a few days or week. That way you do not have information about the tactic it could be a 600 tactic but also a 2700 one. Then you do your best to solve it and your best to trying to understand the tactic.
I find playing unrated at a 600 level is good training. Even if I can get them all, it kind of wakes up the brain to positions that are seen a lot. Of course once I get bored or something I can always raise the rating to 700 etc.... And that's aside from playing rated every now and then. It seems to be good practice at my level. What does everyone else think about this approach.