I do have a question about what you said. When I measured, it was before the boil and I was already 40 points lower than anticipated. How would you correct that? If I boiled longer, would it increase or decrease og?
Right, you're not going to correct a 40-point deficiency easily. You'd have to boil a lot. I have done just that in the past; you will end up with less beer than you planned though. I have an electric system, so boiling longer is not really a problem.
When you boil, the water leaves in the form of vapor, but the sugars do not. Thus, the gravity increases.