For me it's hard to quantify. I have a set of cymbals that work well together and each does several different things well. I'm very familiar with what each cymbal can and can't do and I have in my head how each cymbal sounds. So when I'm playing a song, it's really more about musical feel and expression, with ergonomics factored in there to a degree. I generally have little idea ahead of time what cymbal I'm going to use where... other than like whether the tune starts out on the high hat or a ride cymbal. I'd say it's like your cymbals are part of your musical vocabulary and you just use them accordingly. Generally speaking, if I think too much about what I'm going to play it usually doesn't turn out as good as if I instinctively play what I feel. I know that probably doesn't help all that much.