Calculation Logic

I am trying to add means to add a upcharge (reverse discount) to a donation form for a charitable organization that will add a fixed percentage of the donation amount to a total if the user elects to do so using a textbox. Unfortunately constant value (eg. numbers) are not allowed in calculation formulas (a good idea to add this). So I created a separate number-field for the discount percentage ( =0.022) and used it in the formula. I also have a value for the checkbox ( =1.0 ). The formula looks like this:

{donation} + ( {check-box} * ( {donation} * {number-field} ) )

Everything seems to work as long as the number-field is displayed in the form.. the total calculation is increase by 2.2% but when I hide the number-field using a condition… the calculation comes out incorrectly to double the donation. I found something similar in another topic (https://wordpress.org/support/topic/forminator-and-coupon-code/) but here the amount is only added in the formula not multiplied…. any idea what is going on here.