One more convert to the wonderful world of stretch! Yay!
As a starting point, about -10% negative ease in all directions is a good place to start with most lyrca, but most commercial leotard patterns will have some negative ease already built in so you might not need to worry about it. I would measure the pattern itself, too, as I think some of the commercial ones come up big.
If you want to figure it yourself, multiply the body measurement by .9 - so 100 x .9 = 90
Re the other post about stones, I second GemTac. If you are getting glue rings with hotfix, you are probably leaving the stones in the tool too long.