You seem to be on the right track/fine.
since your group averages are not fancy you can also just have used the insert summary function (Blue sigma button) to get both of the values you wanted.
use the insert summary function button twice.
Field to summarize = @Days formula field,
set the calculate this summary = average.
for the group, summary location= select the group.
for the report tota,l summary location= select the Grand total/report footer
However what you did with the RT and the Formula field do the same thing.
Edited by DBlank - 18 Dec 2013 at 8:35am