Override calculated field

I am trying to create a form that has calculated fields running in a column. As the fields calculate by row, and then are totaled by column (see below). Everythin works fine when the user places a value in heder 1 and then header2. However, they want to be able to override the calculationn and just enter a number in Header 3. When that is done, the number appears in the total field, but not in FieldC* For the Field C and the total field I used the "Value is."

Here is what it looks like:

Can someone tell me if is possible to override the calculation when a user directly enters the ammountin FieldC and if so, how do I do that?