Hi Brian,
Well, it's a good question. And the answer to all such "good questions" is, it depends. My last PA plane was about 39oz, ready to finish. I thought, man, that gives me like a 15 oz finish budget so I didn't worry too much about it. I ended up putting something like 24oz on in paint and the plane was a pig and flew like an anvil.
The current plane came out a bit porky on the basic airframe. About 48oz ready to fly (it's only 590 square inches). The target weight was suppose to be about 55oz. So I was really careful with the finish. The base color is a bit thin (transparent), but not too bad. And the final finish weight is about 6oz. It's very easy to add weight during finishing. Just get off as much filler as you can and go easy on the trigger with the colors and top coat and you'll be fine.