That is an excellent question. The way this plugin works, it queries your database to pull out all of the purchases of the item being looked at. It gets the total amount of sales (donations) of that product and then makes that a percentage of the goal amount. It does ignore purchases of other products, taxes and shipping in the total.
You could add in a multiplier to change the actual amount to be percentage of what’s given. But that would affect all products and it would have to be the same percentage too, even for variable ones. If you want to go that route, let me know and I can show you how. Sorry there isn’t a way to create more specific control over that.