I agree. Percentage changes are usually wrong, and I have yet to see a case where it isn’t. I wrote the script because some people insisted.
You want to reduce spacing? Then subtract x number of units from all the sidebearings. You want to increase spacing? Then add x number of units.
A percentage change is not ‘increasing’ or ‘reducing’ sidebearings. It is de facto randomly changing the spacing. Some glyphs will receive a lot more, some will receive a little more, some remain unchanged, some are reduced a little and some are reduced a lot.
I would be very interested in a scenario where this would actually make sense. I do not believe it exists though.