65 Dart GT - automatic transmission fluid level

Son drove home from college today and said the transmission had been shifting strangely the past few weeks. I warmed up the car put in neutral and checked the fluid. It was low.
I added about 20 ounces of ATF, drove it around for a few minutes, back into neutral. Son checked it this time and level was at the "add 1 pint" mark. He added a pint (16 oz.) of ATF, drove it around, back into neutral, checked again. Still showing to be at the "add 1 pint" mark.
While wanting this to be teaching moment, also don't want to over-fill the transmission, not sure how much more it needs before it hits the full mark. Please advise.
Hot while in neutral and then fill accordingly. That low would explain what was going on.

Jake