What you describe in the final lines of your report sounds like a summary report where you group your records by "DBH Class" (Have no idea from your post what that is) and use summary fields and sub summary layout parts to produce the needed sub totals (sub averages if you will) and the format you describe.
But I can't really picture exactly how you have currently structured your data--especially how a "self join" would be used in this case.
A DBH 'class' is a bit of a forestry techie term. It means all DBH's that are the same. So all trees that are 45 DBH are said to be in DBH class 45.
I had got a bit farther along after I posted my question, using what you later suggested.
Thanks for your help.