Is there a linear relationship between gearing and crank arm length? For example, on my multispeed bike, my favorite gearing is 34/20 which is 1.7 and with 26" tires, that is 44.2 gear inches but I have 170mm crank arms. For my wife to be, I want a similar "feel" (pedal effort) but her bike has 24" diameter tires and 140mm crank arms. So does that mean instead of shooting for 44.2 gear inches, I should also compensate for the shorter crank arms and instead go for (140/170) * 44.2 gear inches = 36.4 gear inches? If so, since they are 24" tires, I will need about a 1.5 gearing so something like 34/22 or 32/22 sprocket combination should be just about right.
Additional information (although not sure if relevant here) is I am 5' 9" tall and she is 4' 11" tall. With her shorter, weaker legs, I am thinking a little extra compensation is in order. The "shortest" gearing I can go with (using sprockets I already have), are 28/22 which is 1.27 which will be 30.5 gear inches.