Heres a straightforward way of replacing the rest of the fluid while the pan is off
Before replacing the filter seal,, insert a hose into the hole where the filter goes and put the other end in a 6qt container of new fluid. Make sure the end at the trans is sealed and the hose length is as short as is practical. Turn the engine over until the fluid is just about gone. You should be seeing clean fluid falling into the catch pan. Make sure the engine doesnt start. Theres very little old fluid left at this point, less than a quart. Now finish up as you normally would.