I have the same issue with different boil off rates, and mine used to be as much as a 1.5 gallon difference. I've ended up just adjusting my boil off rate for the weather, and I've created 'winter' and 'summer' profiles on beersmith to help compensate. Now i'm usually within a half gallon...
Brewed a great ESB. Decided to cold crash it, so dropped the fermentation chamber temp down to 36. Forgot to take the blowoff tube off of the airlock, so I ended up sucking the 2 qt bucket of starsan back into the carboy. That batch is now referred to as Extra Starsan Bitter.
You're essentially correct about the OG and FG. Runoff gravity is just that; it's the gravity of the runoff from the sparge, normally a fly sparge. So it's really a snapshot of the amount of sugar you're still getting from your mash at that point in your sparge and an indicator of when you...
Sorry, should have mentioned that I did add the proper amount of priming sugar. Also, after priming and bottling, I stored the bottles at about 73 degrees.