Calculationg & Avg time in a report

Access 2000

Each record has several time fields (short time format). I'm using
the following 2 Textboxes to determine actual "Scene Time"

[txtTotalTime]=DateDiff("n",[TimeArrivedScene],[TimeLeftScene])
[txtTotalSceneTime]=[txtTotalTime]\60 & Format([txtTotalTime] Mod
60,"\:00")

The field in the first calculation [TimeLeftScene] could be blank at
times.

All of the above is in the detail section.

What I need now is an Average Scene Time for the entire report.

Any & All suggestions are truly appreciated!

Thanks
Dale