What is the best way to perform advanced calculations with Gravity Form, please?

Then you will want to set the product amount to be set as the 10% and have the Stripe Add-On take the payment just for that. I would add a html field in your form to explain what's going on and if the price is different per customer, I'd use php to calculate what that 10% discount is and then pass to the form when you call it (in php). Unfortunately the html field cannot be pre-populated so usually I use JS to change the html field content for the user. If that content also needs to be changed on a user-by-user basis, have JS AJAX to get the new total Then concat that response into the html.