As Brian said it doesn't take much to overfill these trans. Since mine has a leak I made the mistake of adding about 1/2 a quart when it was at the 1/4 mark. Just a little overfilled!
Anyway make sure that MB fluid is compatible with the older trans. It isn't that new stuff that all the sealed for life trans use? I don't know if that stuff is backwards compatible.
I think I added 6.5 quarts total last time I changed it.