Originally Posted by dcp27
Before you top off with water for the OG (you can calculate the OG to the topped off amount). You can take the pre-boil gravity just before the boil or at the start. and the FG after 2+ weeks
I was going to mention pre-boil grav. The beauty of it is that you can do it, and dump your sample right back into the boil. PBG will tell you what your extraction efficiency is. http://hbd.org/uchima/tech/efficiency.html
That's important to know, because it will tell you how well your mashing procedure is extracting the (theoretically) available sugars from the grain. Once you know that, you can calculate your expected OG accounting for boil-off.
There's a good calc for figuring ext eff here: http://www.brewersfriend.com/brewhouse-efficiency/
But dcp is right..bite the bullet and run hydrometer readings for a couple or three batches to see what you're getting. It's worth the waste, meaning your beer will be worth the wait.