hmm. ok, i think it would just be a guess though. Like I may stop kegging at 1.5 G, but it's difficult to guess how much of that 1.5 G is trub or not.
Also, I would typically not rack to a bottling bucket when I keg, just siphon from my secondary (or primary)...so how would I add the priming sugar?
Would I possibly keg what I think I want to keg, then siphon the rest off into a sanitized bottling bucket, in that situation I could easily determine the amount of beer I am bottling...that seems like a lot of work but I guess it's what I need to do?