I'm not aware of anything in the building code that defines what you ask. That may be addressed somewher in toilet industry manufacturing standards, but I'm not familiar with those...and for that I'm greatful.
In my opinion, the best flushing toilets are made by Toto.
Years ago I came across a web page rating low-flow toilets...I thought it was by a guy named "Terry Love", or something similar...try an internet search with his name. If you come up dry, post here and I'll try to follow-up.