After a night of thought and reading others thoughts here's what I got.
Do you want to measure P&G by heart rate? You could get a monitor with current and average BPM.
It would probably be best to do these tests in the most upright, comfortable riding position you can. That way you are less likely to skew the results with better or worse body position. The numbers won't be the same but the measurable effect will be the same.
There are also physiological reasons to keep your legs moving while on the bike--like transport of nutrients and wastes to and from the muscles, and keeping the muscles warm.
I'd love to know the answer
but there are sure a lot of variables to deal with.