Dynamic GravityForms product option (Instructions only)

I'm using the WooCommerce (WC) Gravity Forms (GF) plugin so that I can use the flexibility of GF in my WC products, and I have a need for a dynamic product option. Specifically, it's a charge that has a daily rate, so the total of the option is based on the number of days selected. I already have a field that automatically counts the days, but I don't know how to add this into the total. I think it's possible to dynamically populate a product option with the value of the text box that's already calculating what the option's dollar value should be, but that's beyond my skill level. And I'd rather defer to someone who's worked with this kind of issue in GF before. Please note that only instructions (in a document) are being requested, not any hands-on work because I need to actually know how to do this so that I can apply it to other things (as opposed to having somebody do it for me every time).