I am not much of an engineer when it comes to my crocheting so it took me a while of trial-and-error before I learned how to get my circles flat. And until very recently it was still a "let's see, does this need an increase?" process. However, I have recently found this magic formula of circle-building and it works very well, so I thought I might share it with you:
1. Single crochet circle - 6 increases per round
2. Half-double crochet circle - 8 increases per round
3. Double crochet circle - 12 increases per round
It is of course also important to space your increases evenly (for example every 2nd, 3rd or 4th stitch) to make sure that the circle stays round and does not "grow a bump" on one side - unless that's the idea!