I don’t notice an appreciable difference between 5 versus 10 gallon batches. The only way to know for sure is to fill up to your estimated boil volume and boil it for an hour.
I target 14 gallons… I expect some loss due to the very fine trub, yeast, handling… And most important, sampling!
If want to cut it so close, you’ll be disappointed… Brew to what you expect an oversized batch will make… Then, if its too much… your a winner of a chicken dinner!..Sneezles61
Kent, since you are brewing a 10 gallon batch in the same BK as you did the 5 gallon batch, I would expect the same boil off rate because the surface area and boil time would be the same.