Calculating total

I have a form with 2 columns containing 6 rows. Column 1 is names Diesel and column 2 is named DEF. I am trying to calculated the total for both Diesel and DEF. However, I need to add 0.75 to the total for each row that contains an amount in either Diesel or DEF. This additional amount would still only be 0.75 even if there is an amount in both fields of the same row.