Sounds like the easiest way to keep it fresh. What did you do, first time you drained it you measured the amount that came out and then just added that amount back in every time you did a Oil and Filter change and transmission pan drain?
I just added till it was within range on the dipstick, that comes out to almost four qts. Always check your level.
BTW: I have been using dextron III for the last four years with no issues.
Dan