To create a custom calculation select the field that you want to have a custom calculation and in the “Price Type” option select “Formula”
The formula builder will show up, in this formula builder you can do any arithmetical operation using fields in your form. For example if you want the price to be the “Product Price” field multiplied by the “Quantity” field your formula would look like this:
You can also do much more advanced calculations, like calculations that use conditions or attributes in your product.