Tranny fluid level changes considerably depending on the temperature. The fluid level should be set with the fluid at 80C in order to get the level correct. This tranny is very sensitive to the fluid level, so I (and I do almost all my own service) take it to the dealer for this particular job. Most charge 1.0 to 1.5 hours labour plus parts.
I abhor the dealers myself, but use them for two jobs: Tranny fluid and coolant change.
1998 C230 "Black Betty"