Unable to change field settings in calculated field in a pivot tab
 

I have a pivot table that uses a calculated field in the data items section.
This is just a simple calculated field that divides one column by the other
from the data list.

The calculated field works fine, except that I need the data averaged and
not summed as it is defaulting to. In the field settings for the calculated
field, the ability to change the "summarized by" block is greyed out and I
can't change it to average.

I know of a work around to this by just adding another column to my data
list but I would rather not do it that way.

Is it possible to change the "summarized by" setting for a calculated field?

Thanks in advance for any help.