@Waylander
You just copy FONT_RAMP file to root (= top level) of Kindle drive and then restart your Kindle and that is it. Then just use Aa menu as always. You tweak the values, copy the file, restart Kindle and test the results on your Kindle untill you find the sweet spot for you.
One thing to keep in mind: FONT_RAMP thing is in effect only for books in KF8 (= azw3) format, mobi and KFX would still used default/factory values.
If your Kindle is jailbreak-ed and you are comfortable with doing a tiny edit of a single .jar you can redefine font sizes for books in KFX format :
https://www.mobileread.com/forums/sh...postcount=1850
You would have to repeat that edit after every Kindle firmware update if you don't prevent your Kindle from updating.
edit: Here are the default font values:
https://www.mobileread.com/forums/sh...&postcount=153