As a general rule, I normally do just put the "change" amount back in. On this particular job (day before yesterday) the owner was having some shift problems. The ATF wasn't burnt smelling, but was quite black, and a little low for some reason (no leaks noted). I proceeded with a ATF change and wanted to be sure of the level. Initially, I put in the 5.8, then roadtested. made a minor correction to the throttle valve cable, seemed to work great. Level, even though I had the trans oil quite hot, was still a little low, so I added a little. But usually the "change" amount listed will give the proper reading on the stick.