Depending of course on factors like gear (if so what type) and no gear, and your definition of ATG…I consider the difference between “parallel” and IPF depth to be ~3 inches.
Raw for me between “parallel” and IPF depth about 30lbs.
In most people, A2G is probably only an inch deeper than parallel.
Unless you are on olympic lifter, if you say you are doing “A2G” squats, it’s probably not truly A2G. In fact, it’s probably barely parallel.
Parallel means the crease of the hip is at the same level as the top of the knees.
For most lifters, a A2G or BD2F squat is essentially the same as a parallel squat. If someone is claiming a big difference, it is because they have lousy depth on the “parallel” squat.
I’ve found I could parallel back squat about 20 pounds more than I can squat ATG.
however, this is with the same stance and technique. If I changed my footwear and stance and did more of a “power squat” variation, I would think the difference would easily be 40+ pounds raw.
But I’m squatting medium narrow and atg like more of an olympic style, so I think that would result in a bigger difference because the leverage is worse.