I'll start out by saying I'm completely new to kegging (have bottled my fair share), but I've read a ton on this site and learned a lot (thanks!).
I did a ~15-gallon AG batch that got split into four 5-gallon carboys. My share was three, one went to my buddy. So each carboy had about 3.5 - 4 gallons of liquid gold in them. I fermented and dry-hopped (3 weeks total), then cold crashed for a day at ~35F. I kegged two of the carboys into their own 5-gallon corny, so it wasn't completely full of liquid, probably about 3.5 gallons got into each corny, so quite a bit of head space (which I purged with CO2).
Target CO2 volume was 2.3 (referenced http://www.kegerators.com/carbonation-table.php
). I have a 5 pound CO2 tank. So to carb, I put 30psi on, rotated the corny upside down until I heard the CO2 stop, then rotated again, and repeated for about 4-5 minutes. I let the corny sit at ~35F for roughly 36 hours with gas on at 30psi. I periodically switched the gas between the two kegs. Then I brought the pressure on the corny down and let it sit on gas for a day at 10psi at ~40F. I then drew some liquid gold from my picnic tap (at 10psi) that is on a 6ft line and got foam. So I dialed it back to 8psi and got a good pour, but, it is a bit over-carbed. And, my initially full 5# CO2 tank read 1000 when I started, but now reads 500.
1 - Is that much CO2 usage typical (I sprayed to check for leaks and found none)? Half of my 5# tank for two 5-gallon kegs seems like a lot to me. Maybe because I had so much headspace in the kegs?
2 - I'm thinking that carb volume is also (along with temp, pressure, and time) a function of the ratio of headspace to liquid. If so, is there a calculation or table that takes that into account?
3 - What could have I done differently?
Any insight and advice appreciated.