Gravity conversion confusion

Mr. Blonde

I've read Dan's questions and answers for hot water heating and he says small, water lubricated circulators are not a good choice for gravity conversion that the B&G series 100 is the better choice. I then found an article by Frank Wilsey on sizing circulators for hot water heating systems that were originally gravity. After figuring all my EDR I came up with 15 gpm. Frank says to use 3.5 foot of head on gravity conversions. The Grundfos 15-42f or 15-58f pump curve seem to be a better match than the B&G, what am I missing?

Dan also recommends primary/secondary piping and controlling the primary pump with an outdoor-air reset control. Could somebody give me a little more detail on the outdoor reset installation and operation

Thanks