You are only replacing what came out, which will probably be 5 to 5.5 quarts if you let it drain down overnight. If the fluid level is correct now, measure what comes out, put slightly less back in, and you should be close to right on. Rememebr that there is only about 1/4 quart betwen the high and low marks on the dipstick, so add more only a little bit at a time. If you want, you can drain and refill the fluid a second time after you've driven a couple days, and you'll have 3/4 fresh fluid in the transmission rather than 1/2 with a single change.
John