I have updated my custom font writing plug-in (Download and extract to your plug-in folder.) to make better use of the kerning information provided in kerning paths. This is useful when you have, or want to make, a custom font that has leading and trailing flourishes which affect the left and right hand margins of the character layers. More information can be found here on Gimp Learn:
https://gimplearn.net/viewtopic.php/GIMP-Python-Fu-Scripts-Plug-ins/Update-to-Custom-Font-Tools-FontSlab-to-Version-0-08?f=3&t=2057
Basically you could have no kerning paths and the letter spacing is dictated by the width of the character layers or a sort of global kerning affecting all instances of a character where the letter spacing is indicated by a path named as the character followed by the '.png' suffix. I have added the facility to use kerning pair paths so that spacing between individual letters can also be adjusted - by a path named with just the two characters.

Here is a plain white custom font that you can use to explore the kerning paths used:

And an example (with a custom font created from the white base) to show the kerning effect - after and before:

Skinnyhouse, you did it again. You do such an improvement on your great script to perfection. Thank you so much for it and for sharing it with us all. I find it to be very useful because some fonts need between letters too much room and it isn't nice. Now this problem is solved.

Thank you Issabella - glad you can make use of it.