Efficiency Question(s)

1. Is efficiency based on pre-boil gravity, or post boil? Seems to me it should be pre-boil as you are measuring your extraction of sugars from the grain, but maybe I am wrong.

2. Yesterday, I brewed a batch of Janet's Brown. I hit my pre-boil graivty right on the expected, 1.055 with 7.5 gals in the kettle. Problem is that my (post-boil) OG measured came in at 1.062 vs the expected 1.068. To add to my frustration, I ended up with only 5.25 gals as opposed to the 5.5 gals I was expecting. When I realized I was short on volume, I expected that my OG might come in higher, but that wasn't the case. Anyone have any ideas on this?

Mash efficiency is based on your pre-boil OG, brewhouse efficiency takes into account your losses to evap/kettle/whirlpool etc and uses the final OG and final volume when it goes into the fermenter. I seem to recall an article on beersmith's website that goes into this in more detail. I think you could also take into account loss in the fermenter as well and use your bottling volume to get your final brewhouse efficiency.

As to your situation on Janet's brown, that seems pretty odd and I have no helpful explanation. I typically hit my preboil OG and water volumes but end up low on the software predicted OG postboil (particularly with beers over 1.060), I just know it's coming and compensate as the calculators relating boil off to increase in OG don't seem to fit my system

Don't forget expansion - you're usually measuring pre-boil volumes hot and post-boil cold. The difference is 1-2 quarts.

Here's a common correction formula:
=Measured Gallons /(1+(Measured Temp-68)*0.000264)