You can use the DAvg Function in the code (check help on that option)
DAvg("[FieldName]","[TableName]","Criteria")
Or, create a group by query and in the weight field select Average, in Sql
it will look like:
Select Avg([FieldName]) As NewFieldName From TableName
I don't know the criteria you are using to specify the range of data, but
you can use the above to do that
