depending on the power the motor is making and how much boost you will be running, for a motor making over 500rwhp you may want to go a little lose on the gap, for something arounf 300 to 400 a bit tighter, always remember the upper ring has the tighter gap with the lower ring being the wider, my car made 567rwhp on 24psi and i gapped my rings at 24 bottom and 21 top, if your making under 450rwhp i would do 20 bottom and 18 top. the loser you go the more blow by you will get from boost getting past the rings so prepare to install a catch can to keep your throttle body clean of oil. race motors always have blow by no matter what you do and it comes boost getting past the rings.
Ive gone .018 top, .020 bottom with 3.820 JE's. Close to 700hp. No problems. To avoid blow by in your intake tract get rid of the PCV circuit. It is not needed.