Originally Posted by HighValleyRanch

I was taught to use the formula:
front teeth divided by the rear teeth x the wheel size to determine the gear inches:
i.e. 50 tooth chainring with a 12 rear for 16" wheels equals. 50 divided by 12 x 16" (as hypothetical as I am not sure of actual Brompton wheel size and depends on tire selection as well)
this gives me 66.66 inch gear.

With the sturmey archer AW 3 speed hub I was using the fixed 2nd gearing plus 33 percent for high and fixed gear minus 25 percent for low. On my 20" 406 wheels I measured actual diameter and came out with 19 inch actual diameter.
I'm sure that there are gear charts for various variables, but this method seemed easier for me.

Open to corrections if my method is in error.

typical old school 27 inch ten speed high gear was usually 52 divided by 14 times 27" giving 100.2 inch gearing