That's correct, Jay. You have the same amount of sugar and and dissolved stuff in the boil before the boil and after. Well, that may not be completely true, but it's close enough so that your gravity readings should not vary significantly. I calculate what I expect my wort volume and gravity to be, based on what I want my final volume and gravity to be, and then hope the beer gods smile on me. That's how I measure my "pre-boil efficiency."