I know it's not as fancy as the online calculators, but if you know your previous best time up a hill it's easy to see how much you'd gain (assuming same power output) by just dividing seconds / weight (lbs/kg doesn't matter).
e.g.
I pre-rode Diablo TT hill climb at 173lbs, with an 19lb bike in 31:30 (1890 seconds). so 1890seconds/192lbs. For the ride I dropped 4.5lbs off my body and maybe 4lbs off the bike (carried no food/water on TT, no cages, no seat bag, no pump, etc..)
Anyway, 1890/192 = 9.84s/lb. Race day weight of -8.5lbs 1 week after pre-ride and I turned in a time 100s faster. Pretty close to 10s/lb, with a little extra for the day-of adrenalin.