Data Grid Custom Math Field

Field Name - Name of the field that will be added to Data Grid fields list 

X to P - Assign fields to variables, only one can be used if needed

Enter Formula - Enter formula with assigned variables to make calculations for your new field. Like (x+y)/2

Example:

Remove TAX from Total - Enter name "preTax". Select "checkTotal" as X. Select "tax" as Y. Enter formula X - Y. Hit insert button. Your field will be added to the end of Data Grid Fields list.