Write someone a check with a lot of zeros?   Of you can create two (2) UDFs and then write a script to take the total multiple it by one of the UDFs and add it to the second UDF. Probably would need a button script. OR if you manually enter the percentage you can have the script activate on the enter of the percentage

Do you need the UDF total to show during Sales Order Entry as the order is entered?  Or after the fact?  It's pretty tricky. A script might be a good way to go.

It might take two UDF's (as Big Louie said), I'm guessing the first one is a new field with the total in it, indicate it is a Business Object based on Totals from Details, Ext. Amount, you can get the total, including sales tax. (tricky, see? maybe you don't want sales tax in there) I don't know if you could then have a second UDF, using the first, and use the expression building on the data source tab to do the calc.  I have not tested it.

This might work for your situation.   Assuming you don't use it for anything else,  use the discount field on the totals tab and enter a negative discount percentage.

It just might work for me. Thank you.

Note that you can use Custom Office to relabel the field

I did this years ago for a client where they wanted to add a surcharge for handling(?) to their bills.

This is a great solution for a, hopefully, transitory problem as material surcharges are becoming an issue for a lot of companies.