Custom metric to show percentage point difference between two scale points

Is there a way to do a custom metric that calculate the percentage point difference between two percentages? We have a high and a low percentage for about 15 items, and I’d like to create something that shows the difference between the two for each item, and whether it’s a positive or negative difference. For example, if one item has 33% for the high and 5% for the low, the difference is +28, but then we have an item that’s 30% high and 40% low, so that difference would be -10. Is there a way to create that custom metric? I’ve tried doing one for each item that takes the high-low, but it’s not quite coming up with that number. In my examples above, instead of 28, it’s 0.28. Thanks in advance!