Okay I've figured it out with a bit of experimenting. It seems pretty obvious in hindsight, but I'll post how it works here in case anybody else is wondering the same;
The formula is applied to SUPPLIER ORIGINAL price to give your STORE PRODUCT price and also,
the formula is applied to the SUPPLIER SALE price to give your STORE SALE price.
The difference (if any) is then calculated and displayed as a percentage.