I’d like to set up a formula that gives the percentage (positive or negative) difference between two numbers.
So in the screenshot above
Row 1: Percentage decrease from 100 to 50
Row 2: Percentage increase from 25 to 50
Row 3: No change between 50 to 50

My formula so far: -(({Starting #} - {End #})/{Starting #})

It seems like your formula should work. If not, you could remove that leading minus sign and subtract the starting # from the end # (instead of the end # from the starting #).

Then, on the formatting tab for your formula, choose “percentage” to display the number as a percentage.