Wheel Studs and Spacers

So, I'm going to have to use spacers for my Dart. There was no way around it. I'll be running a 0.200" spacer (relatively thin) front and back and I was wondering what the general rule of thumb is for needing longer studs. I was told you needs at least 7 turns on the lug nut? Should there be a certain length of threads showing prior to installation of the wheel? Thanks for your help.