I just brew 2-3 batches and then have it filled, noting the quantity required. I can then estimate the amount used per batch. Obviously you could get really detailed and keep track of how long the burner was lit for various operations and get an average burn rate per hour.
I can usually get 4 5gal batches out of a 20lb tank. I use it for everything (heating water for mash and sparge, and 60-90min boil). I am using a Blichmann burner, so I have a relatively high burn rate in exchange for reduced time to boil.
Paul
Brew Pedaler Brewer