I just tried this for my first ever BIAB attempt.
I used 6.5 gallons of water and 1/2 gallons to sparge.
After the hour boil, I was almost 1 gallon short of 5 gallons!
why did I lose so much? There also seems to be a lot of trub too?
How much would I gain by a heavy squeeze?
I MIAB in a cooler and start with 4 to 4.25 gallons of cold water to make 2.5 gallon batches. So if I ever ramp up to 5 gallon batches, I would start with 8 to 8.5 gallons. I've done 75 batches using the same basic MIAB in a cooler process and equipment. It took many batches to dial it in and get the right amounts and learn my system. My mash efficiency is usually in the 80s. Hope that helps.
Edit: Efficiency