My take on the "low" aspect is that it really doesn't matter unless you're riding around parked cars a lot. Even then it doesn't matter unless you're on a trike or a lowracer. After all, drivers always have their eyes fixed on the pavement so they can see potholes. Or, put another way, drivers are always saying they don't see uprights, maybe it's because uprights are too tall! At any rate, I have found that drivers generally give me *more* clearance on a bent than they would on a DF. If you decide in favor of using a flag, the ideal height would be eye height for a driver. Car, or SUV? I can't answer that!

Regarding your stature, it depends on the specific bike/trike. Most bents can adjust for a lot more height variation than that, but bear in mind your father's may have already been adjusted nearly as short as it would accomodate. Adjustable booms can usually be shortened, but it's hard to lower a seat that's too high or move a seat past its adjustment range.