You are on the right path.
After you select your object, click on New. Here you will specify which attribute of that object you want to change. Select the price with btw and calculate using the price without btw. I provided a screenshot for you. Best would be to use this microflow in an on change event on the price without btw field when creating the product. And setting the price with btw to not editable. Hope this helps!
Also, I saw you had an enumeration for the btw percentage. You can use the enum with a decision to split and set that as a decimal, since you can't use the enumeration directly for your calculation. Then use the variable in the calculation. I added a screenshot for reference.