@Silent I've struggled with this for a while. Drip emitters are rated in GPH, but the PR is in in/hr. There's a formula around here that is used to convert GPH to in/hr, but when it comes to the approximation is made to use a 1 sq.ft. are in the assumption. I don't agree with that assumption for my particular scenario, but I'm sure it works for others.
What I did is look at my gallon per minute usage first. I have trees with three 2 GPH heads on each of them, and 4 trees.
Total trees GPH = 3 emitters/tree * 2 GPH/emitter * 4 trees = 24 GPH
Total trees GPM = 24 gal/hr * (1 min/60 gal) = 0.4 gal/min
From this link note that
water usage (gal) = AREA (sq.ft) * PR (in/hr) / 96.25
AREA (sq.ft) = water usage (gal/min) * 96.25 / PR (in/hr)
my PR is set to 0.4 in/hr
AREA (sq.ft) = 0.4 gal/min * 96.25 / (0.4 in/hr) = 96.25 (sq. ft)
This is what I did anyway. The nice thing is this only affects the water used calculation, so if I'm wrong I won't kill any plants or flood my yard.