I just brewed this pliny the elder recipe - http://www.homebrewtalk.com/f69/piney-elder-clone-296940/
Here's what I used for 70% efficiency:
12.8 lb Pale 2-Row 37 1.8 87%
9.4 oz Carapils (Dextrine Malt) 33 1.8 4%
9.4 oz Crystal 45L 34 45 4%
11.8 oz Corn Sugar - Dextrose
My pre-boil volume was about 7.4 gallons, 1.050 gravity
I calculated I should end up at around 1.63 because the corn sugar adds about 8 points, so I thought I was looking OK for my target of 5.5 gallons post boil, a little high (I wanted to hit 1.070 - 1.073)
50 x 7.4 = 370
370 / 5.5 = 1.067
My post boil volume was 4.5, and my post boil gravity 1.070. What the hell?
I didn't leave anything in the kettle. I know there's a lotta hop loss - there's a ton of hops, but I held my hop paint strainer bag over my kettle until I thought all liquid was drained.
And it's like I went from 73% efficiency pre-boil to 62% post-boil.
Any explanation?! It was a 90 minute boil, but that seems like alot of boil off, and my numbers don't seem to add up.