I kerned my font and almost all character combinations got a minus value. It seems very complicated to me to adjust the spacing now. Is there a way (e.g. a tool or plugin) to find the highest common denominator and apply that to the spacing?
What I mean by this is shortening the kernig values to a minimum, because that looks cleaner to me in terms of craftsmanship.
If you have so many kerning pairs, your spacing is most likely not good. You always should make sure that the spacing is perfect and then add some kerning. I fear that you need to remove the kerning, fix the spacing and do the kerning again. Because if you substantially change the spacing, the existing kerning will be broken beyond repair and only make problems.
Make sure to read this Spacing | Glyphs and Kerning | Glyphs
What do you mean by “highest common denominator”? You could try and calculate something like the average of your kerning pairs, and then add that (divided by 2) to the left and right sidebearing of each glyph (as well as, of course, add that value to all your kerning pairs, then also deleting 0-value kerning pairs as well as pairs below x units). This is very easily accomplished with a script.
Go to Glyph > Transform Metrics to bulk-adjust sidebearings of selected glyphs.
Delete the kerning, go back to the font and fix the spacing, do the kerning over again.