0.035 in.-0.040 in. is good to go for a 50-75 shot kit.
just periodically read the cylinder #3 plug. If you experience misfiring from fouling the plugs because of too small of a gap with a plug that is too cold then gap towards the higher end. The problem with nitrous gapping is the fact that your car is running all motor off the button more often than when you are on the button.
the general guideline is that an ignition amplifier allows you to gap wider than a non-amplified system by about 0.004-0.005 in. For every 50 hp nitrous, you should reduce the gap by 0.004 in. For every 75 hp increase, you should go one heat range higher. If you have a 75 shot kit that would warrant a 0.006 in narrower gap but the MSD brings you back 0.004 in. So a gap of around 0.040 in. isn't a bad start.