The way to think about this is to start with an assumption of 100% conversion efficiency. All that means is that you actually convert all starch to sugar in the mash. Simple example:
10lbs grain with a potential of 1.036 PPG. So, after conversion, you have 360 gravity units. Assuming you lose 1/2qt/lb in absorption, your strike would have to be about 30 qts because 30 qts - 5qts absorption = 25 qts (6.25gallons) preboil volume.
Ok, so you have 360 gravity units in the mash spread across 7.5 gallons of liquid/wort. That means the sugar density (SG) is 1.048 or there's 48 units per gallon.
When you drain that tun or remove the grain bag, whichever, about 1.25 gallons will remain in the grain. Therefore, you lose 48 x 1.25 = 60 gravity units. So, now you have 300 units left. 300 is 83% of 360 so 83% is the best efficiency you can hope for.
You may get a little less than 100% conversion efficiency.
You may get more wort to come out of the grain if you use a grain bag and let it drain.
You may get less wort out of the tun if you have deadspace and leave extra wort behind.
I think 80% is a reasonable expectation if you're sure you have good mash pH, a good crush, and don't leave any wort behind other than what is absorbed and non recoverable from the grain.