Average bottom 50 rows of a given group

[beng404]

I have data that is separated by blank rows, ie:

a
a
a

b
b
b
b

c
c

The number of entries in each area varies, but is generally in the 200-400 row range. The begining of each are must be discarded, which I now do just by simple deletion, but I would like to change this. Instead, as the number of rows vary, I would rather just always use the final n (say 50 or so) rows of data. Here is an example of how I currently get my statistics:

For Each r In Columns(1).SpecialCells(xlCellTypeConstants).Areas
    r(r.Count, 9) = WorksheetFunction.Average(r)
Next r

Is there some way to specify the bottom 50 rows of each area?
Thanks!