Nice story. Thank you, I feel as though I've been on the same type of adventure. Every 'used' bicycle has a story.

As far as gearing in concerned, you may want to refer to the "Sheldon Brown Gear Calculator". Whether you deal with 'Gear Inches" or "Gear Ratios" is up to you.

https://www.sheldonbrown.com/gear-calc.html

Just plug in the numbers and get your numbers. I prefer gear inches and my ideal gearing is 60 gear inches. Anything too low I am spinning out, anything higher is too hard to go up hills.

And most vintage European folders I've found have had 60 gear inches. Whereas most modern Asian folders are about 45 gear inches.

Mirfi.