Turn them after fireforming. It's a trial and error deal in my opinion. Sure, you can work out how much to turn off mathematically but it may be quicker to just do it. What you are looking for is a neck wall thickness such that the expander ball can barely be felt when it passes back through the neck. Do it on one case at a time until you get your turner set or you risk trashing all your cases instead of a few that are guaranteed trashed. Cut a skosh, fire the case, and load - repeat on the same case until the result is acceptable. If you overcut and the case won't hold a bullet, start over with another case.
