How do I subtract two fields in a form?

Question

I am working on an expense form and I have everything working except one thing.
We have to subtract any over payments from previous expense reports on the current expense report. I am looking at the calculate tab of the Text Field Properties.  But I do not see a way to subtract my two fields.
I then setup a field that contains a negative one, except when I display the field on the form, the negative one is a positive one.  I did enter -1 as the default value to my "subtraction" field.  I was then going to multiple negative 1 against the the over payment amount so I could get a negative over payment amount.  I was then going to add the negative over payment amount to the total due and arrive at a net due,  But this does not work.
Here is the calculation I am trying to do:
Total Expenses     (calculated field from all of the preceding fields.)
Deductible Expense       (user entered)
Net Expense check          (Total Expense - Deductible expense).
Can anyone tell me how to do this?
thanks,
Dennis

Answer 1

With Acrobat there are 3 calculations options.
"The field is the ____ of the following fields:" options are for sum, product, average, min, and max operations or for actions that could be repeated over multiple fields. One does not usually subtract or divide multiple fields in on action.
"Simplified field notation" uses only field names and values. Field names must start with an alphabetic character and should not have any spaces or special characters. Cannot perform conditional control statements or functions. Special characters or spaces can be in field names if the JavaScript escape character precedes the special character or space. The 4 arithmetic operations can be performed.
"Custom JavaScript calculation" has no restrictions on field names, control statements, functions or mathematical operations. One needs to access the field object and the value property of the field object.