For formulas there are many different kinds of Col formula combinations you can use, Jim suggested one and here is another which might work (add Abs/swap order as needed)

`Col Mean(If(:EPG == "TE", :Value, .), :Coupon, :Site, :"Abs[Voltage]"n) - Col Mean(If(:EPG == "TP", :Value, .), :Coupon, :Site, :"Abs[Voltage]"n);`

I know you asked for a formula, but if you can split your data this is much simpler to do as you can just add new difference column as you would be comparing row by row. In this case you might want to remove duplicate values first to avoid some possible issues. Depending on your data you can do this from rows menu and select duplicates -> remove them or create new summary table.

-Jarmo