I am trying to chart daily telephone hold times that are measured in minutes
and seconds (mm:ss) and then a cumulative sum for the day in hundreds of
minutes and seconds (mmm:ss).
I've tried using the custom number options, but they all seem to default the
cell entry to a format I do not want. It seems I have to create my own
formula, but haven't had success with that either. Any suggestions?

(Also, if for the sake of easier data entry could be set up as mmm.ss that
would be wonderful.)
Thanks!