Figure on buying 2 gallons of ATF if you are dropping the pan and replacing the filter. Button it back up, add a gallon, fire it up, let it get warm, put it in all the gears for a few seconds and then check. You will probably end up adding in another 2 quarts or real close to that amount. Now, you have 1-2 quarts left over to top off the power steering system, tcase and your other vehicles.
Don't understand all the fear of exactly how much to put in. The enemy of these systems is HEAT, caused by friction. You won't hurt anything being a little over full. I'd rather be a little full, than a little empty. A few QUARTS too full, yeah you over did it and need to get some back out. The engine and transmission don't like the extra pressure, and will blow seals out.
Transmission and engine oil are the two that require some sense of how much to put in. Its pretty hard to overfill the differentials, unless you went in through the breather or something. Same idea with the T-case, fill it till the hole. Hard to overfill the power steering, it will very obviously overflow when the cap is put back in.
If you take the top fill plug out of your transfer case, and fluid starts shooting out...you have a T-case to transmission gasket problem. Thats the only reason I can think of that would cause a transmission to take more than 'recommended'. Other than that, fill stuff till its full and to spec.