I don't think you're doing anything wrong, just low efficiency. Based on that grain bill your estimations are wrong. On average base malts give you roughly 36/ppg (points per pound per gallon).
So 20lbs * 36ppg = 720 + 34 (estimating the crystal malt) = 754
Divide that by your pre-boil volume (8.5 gallons) = 88.7
So your pre-boil gravity at 100% is 1.089 or 1.088 depending whether you round up or down
Given your measured pre-boil gravity, your efficiency was roughly 59% (According to beersmith 55%)
Forgot to mention the 1lb of D90 in the recipe, which would bump up the post-boil OG. I just wonder why my efficiency is so low.
Have you brewed beer at this OG before? High gravity mashes tend to be less efficient anyway.
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I haven't, this was the first time I brewed something over 80ish.
Quality of crush is always a good place to start followed by how well you doughed in. Also, how well did you stir the mash prior to first runnings and again at sparge, you really need to stir like hell to knock all the sugars loose.
Then there would be the mash pH to consider as well for proper conversion
I ran the grain through the mill twice, and got what I thought was a good crush. I didn't stir prior to the first runnings, maybe that's part of the problem? Prior to the second runnings I stirred pretty well, and for the pH I followed the additions per Bru'n water but I didn't measure.