You have the Noritz right? Hold up & down arrow (on keypad) at the same time and you enter a list of service codes. Scroll with up arrow to 31 & it will tell you precisely the outgoing GPM's. It can tell you a bunch of other cool things too. I'll get the list and post what the codes are..
It can tell you the exact temp of the water leaving the hot water out. When I set my temp to 180 and ran it, there are moments when the heater has heated the water to 195+ - for brief moments, then it settles back to 180 on the nose.
Cool! I never bothered reading the manual...