Help: datavision formula using subtotal field

Please, I am trying to make an formula in DataVision that divides a
subtotal field into one report.row (in order to obtain an average),
I have tried some things, without sucess:

{%sum{DB.DATAFIELD}} / {report.row}
sum{DB>DATAFIELD} / {report.row}

Thanks in advance for any help.

Luis Cáceres.