Curtis,
i just do it the way the owners manual says.
wait for about a 25deg spread between thermometers....take a reading....determine the correction factor.
they dont say to use the temp correction for determining the bench correction factor, so i never have.

since the reading has always been 80%, i rarely check the bench anymore.
i mostly check the calibration if ive tested a head and the numbers seem off from what i was expecting.
then i'll stick the orifice plate on to verify the bench isnt doing something "strange".
it's never been a problem with the bench.....it always checks at 80%.

to my way of thinking....the orifice plate just becomes the test piece.....and on my bench anyway, the repeatability for that particular test piece has always been right on the money.
regardless of whether you should use a temp correction for testing the orifice plate or not, the fact is the bench always shows it as 80% flow.
i'm satisfied with that kind of repeatability.