First off, you have NULL value when you are dealing with numbers and a wish to sum them. 

Why then, isn't each deduction on the detail line a formula itself? 

If isnull({MED}) then 0 else {MED}  // will equal a number

then your formula to subtract deductions will just be a simple math formula.

{AMNT}-{@MED}-{@TAXWH}  //@MED is the formula not the data field

that's the simple way(but uses more formulas)...I am sure there is a more complex formula method to include the null check using variables and an IIF statement all wrapped into one neat formula block of code.

hope that helps