How much PSI when using custom wheels?

You should set your air pressure based on the load rating of the tire; has nothing to do with the rim. Start with the recommended pressures on your door-jamb (or owner's manual). Whatever tire/rim you switched to, it's still carrying the same car.

If you really want to optimize your tire pressures, you need a controled area where you can get them up to operating temperature, and then immediately read the temps (using a dedicated tire pyrometer) in the inner, center and outer portion of the tread. For each tire. Colder in the center would indicate under-inflation, or vise-versa. Variances of the inner to outer temps are an indication of camber and scrub, but that's another can of worms, outside of your original question.

Remember, a few pounds difference between front/rear tires can transform the handling from understeer to oversteer, so be mindful of what you're doing, and it's best to have a safe place to test these new limits of adhesion. Trees and ditches are pretty unforgiving.