Since most of the cars which I have been able to afford over the years have been very thoroughly used by the time I bought them, I've often had to deal with some ATF leakage, as is the case on my current '85 300TD (333,357 miles). I have adopted the practice of getting the fluid level to the exactly correct level by the factory method and then parking each car in its usual place on our slightly sloping driveway and checking the level again with everything cold. I make an extra mark at the resulting level so that I can do a quick check in the morning anytime I feel insecure about the ATF level, which is often. I never add any, however, except after rechecking with it all at the proper temperature, car as level as possible, etc., etc..