How to use calculation and which one to use!

I am wanting to add together 2 x donation amounts then minus $50 (see uploaded screenshot) and have the total in the 'Your Donation' box (at bottom of page) but not sure which calculation method to use and how to set it up.

Here's what you need to do next.

First, add a Form Calculation widget in your form.

Second, have all the Donation fields in your form that should be included in the calculation added to the Form Calculation widget.

Please refer to this guide for more information on how you can do that — How-to-Perform-Form-Calculation-Using-a-Widget

Third, pass the value of the Form Calculation widget to the PayPal Express payment field in your form. You can do that by following the instructions in this guide — How-to-pass-a-calculation-to-a-payment-field

And lastly, hide the Form Calculation widget in your form. — How-to-Hide-Form-Fields

Please test your form after making the changes mentioned above. If, in any case, you have questions, please do not hesitate to let us know and we will be glad to help you.