It is not necessary to write code to do what you want. In Row 1 for
each of your columns, create  a title. Select A1:C2
From the data  menu, select Forms. The form dialog will show you the
names of your "fields", you can perform your data entry and as you move
to the next new record, the Form will automatically copy the formula
located in column C down to the next record. You will not be able to
see the result of the formula  until you leave the record, but the form
will show you the result as well. I believe the limit on the number of
fields you can use with Forms is 52.