Fee formula [FIELD_VALUE] and math operator

Hi,
I do not understand how to correctly insert the field [FIELD_VALUE] in the Fee formula
example
if I have two fields named first field is "cost" as the second field "VAT" and I have to sum
is the right place [cost] + [VAT] in fee formula?

Maybe you can send me the link to the form you are trying to setup so that I can understand and guide you.

Tuan