I see what you mean about flaked grains. They're unfermentable on their own, but they borrow the enzymes from malted grains they're mashed with to convert the starches to sugars. I've long heard that darker crystal/caramel malts are usually less than 20% fermentable, chocolate malt is also under 20%, and roasted malt was either at 0% or close to 0%. But obviously not everything I hear or read is necessarily accurate.
I wouldn't be disappointed if it turned out to be around 1.020 or so, but that would mean only 60% apparent attenuation, whereas I'm guessing maybe more like 70%? (around 1.015 or so?) I won't know for sure until I take the final gravity reading, though.
EDIT: One thing I also kind of wonder about is the effect of the 70C (158F) temperature in the mash. I know some people do really short mashes, whereas I had the mash temp at 68C (154F) for the first 30 minutes of the mash. It was around 66.5C or so a little before the 30 minute mark, so I increased the mash temp, but overshot it a bit and hit 70C. I know that some people only do 30 minute mashes anyway, and it's my understanding that higher mash temps mean faster saccharification, so I think it broke down to something like: first 30 minutes at 68C (with the latter part around 67C), next 10 minutes at 70C and 69C, last 20 minutes at 68C. I know there are those out there who have done experiments on mash temperature and mash time to test it out, plus there's been a lot of research into the subject, but it does make me wonder.