What changes can be made to the fast formula for the following scenario?

I have created a deduction element with input type as Flat amount, proceeded with element link creation.

I have used this fast formula TAX_DEDUCTION_FLAT_AMOUNT

What needs to be changed in the fast formula query so that the input value entered will get reduced by 100 and the remaining flat amount (deduction amount) is subtracted from the pay value and gets reflected in the Quick pay?

For example: If we enter the input value (flat amount) as 500, in the quick pay the deduction amount should be 400.