how to add math formula in design view for finding percent

[eeps24]

In my report I have a PO price and an Invoice price of an item. I need to also include the percent difference in price between these two. How can I do that in design view?
How can I incorporate the formula in here to calculate the price difference in percentage form between the two?

Isn't the method to subtract both #'s, then divide answer by PO price then multiple by 100? Not sure how I would write that in here? The 2nd screen shot is my report exported to excel. You can see I have PO and invoice prices. I just need the difference as a percent.

[June7]

This is difficult in a query. Requires a subquery to generate the PO totals.

Easiest done on a report set up with Grouping & Sorting and aggregate calcs in footer sections.