It'll work, but it relies on the curve being correct, which is iffy (i.e. if your voltages are off, the actual curve won't match, inducing some kind of error).
I also prefer a differential gauge; it's just simpler, and reduces the error.
Make sure that whatever you end up using has a decent resolution; if the graduations (analog) or resolution (digital) are +/- 1 psi, it'll be pretty useless.
I use convert-me.com to convert psi's into something I can relate to.
A flowmeter is really simpler. I picked up one on eBay dirt cheap. Otherwise the best I've seen yet is the Swissflow unit.
http://wbta.us/forums/index.php?showtopic=57