i have a similar question, but i'm bottling, not kegging.
i plan to bottle an IPA that i have been lagering at 50F in tertiary. i didn't have time to bottle after dry-hopping in a secondary, so i put it in a carboy and put it in the fridge at 50F.
using my widget for priming i need 2.15 oz. corn sugar for 5G to target 2 vol. CO2.
that's for 5G of beer at 50F- which has 1.2vol. residual CO2 in it. this is the first time i've lagered before bottling. when the beer warms towards room temp as i'm bottling, and is at room temp while conditioning, which temp should i plug into the calculator? at 70F, 5G beer has only .83 residual vol. CO2. that would call for 3.13oz. corn sugar.
that's a huge variance.
i've had issues getting consistant carbonation when bottling. i want to get this right. i'd rather err on the side of slighty undercarbed than over.