Change tip percentage for gross

How can I change net sale (with discount included) Tip percentage to gross sales amount?  Net vs Gross. I've  I have been trying to understand why my tipping has been really low all of my new clients receive a 25% discount so at the end of the sale I've noticed that the tipping option gives a percentage but the percentages going for the net value instead of the gross value. How can I change that?

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