So, just going through the numbers you posted, it looks like your system performed exactly as you expected.

You state an actual preboil volume of 3.4 gallons and SG of 1.020. That becomes a total of 68 gravity points, which divided by grist weight means you extracted 1.026/lb. This equates to 68% to 70% mash efficiency which allows some margin of error, but matches what you stated.

However, if you stated a BREWHOUSE efficiency of 69%, then that is simply not possible because your losses are just about equal to your yield. So, even if you have a mash efficiency of 100%, you still can't have a brewhouse yield of more than 51%.

The 35% efficiency number BS gave you is the calculated efficiency on the Fermentation tab, correct? Let's remember that this is the percentage or total available sugars that made it from the grain into the fermenter. Since you're already losing half the sugar to trub, the remainder is your mash efficiency difference.

A simplified version of figuring your brewhouse efficiency is the following formula:

BHE = (Sg x BV) / (Pg x GW)

BHE: Brew House Efficiency

Sg: Wort Specific Gravity, post chill (or post boil)

BV: Batch Volume, as measured in the fermenter

Pg: Potential gravity of grain

GW: Grain Weight

Based on the information you provided I estimated the post boil gravity. That means you got:

35% = (26.6 x 1.3) / (38 x 2.6)

I've cross posted this to the other forum, too.