With your wheel bolted on the car with a stock lug nut, measure how much stud is protruding out of the nut. It is safe to run a spacer equal to that measurement.
I wouldn't run anything that subtracts the amount of threads your lug nuts will grab. 5mm should work just fine depending on your wheel flange thickness.
The most important part is to keep everything hubcentric and wheel centric. I bought my spacer/adapters from Wheel Adapters, Wheel Spacers, Hub Rings, and much more! | Motorsport Tech They make each set exactly to your specs, hub centric and wheel centric 3-30mm thick. Check them out before you buy anywhere else! They built my adapters (25mm and 54mm) and they couldn't fit any better and keep the wheel perfectly centered. No, I am not affiliated with them, just extremely happy with their product!