You are correct that there is a direct relationship between BB drop and crank length vis-a-vis pedal strike clearance, but also other factors such as tire height, pedal width / height, Q-factor and even cornering technique. Given your leg length it makes sense to use longer cranks, although I'd still go about 5mm shorter on your FG cranks than on the geared / freewheel bikes. It may seem a little ridiculous, but in your case I'd consider 27" wheels, just as with me I'm better off with 650 c/b or 26" wheels, although I've yet to build a FG with 650c wheels or get cranks shorter than 165mm.