well - in a piece of circular knitting you usually increase 8 stitches every other row.

1-8
2-8
3-16
5-24
7-32
etc.

as I said - there is some fudge factor. instead of doing regular increases I am inserting a design that both increases and decreases the number of stitches - but when I've stopped and checked to see if I need to change something to come out close to the theoretical I've been right on the mark. Haven't had to apply a fudge factor! (Too few stitches and you end up with a bowl shaped piece -the edge pulls in -- too many and the edge turns into ruffles