Like he said, eyeball it... Most of the time it's 1:1 with things like Omni, etc. High end DBC PPG paint is 4:1. I'd say add the paint, a little hardener and some reducer. Reducer all depends on how much you need it to run out, etc. Some people don't even use hardener, I know that when I do small touch ups I don't use hardener in the paint and it does just fine, I only add it to the clear.