IIRC, margins were stored in a device-independent manner, which meant the scale was DPI-based, and I never actually groked the formula.
You can look at the (old, old) margin tweaking hack for Kindle, and then dig into the highly obfuscated bytecode to see if you can fish something useful out of that.
Or, you can simply take a snapshot, then pull out the ruler tool in any image editing software, and you'll have a pixel value for your device, which you can then translate to a KOReader value (the scaling formula:
https://github.com/koreader/koreader....lua#L278-L300)
(The margins recap popup also translates that to px for you).