At the risk of taking the thrill of the eBay hunt out of this: if your present external rangefinder has the usual dial that you turn, with the meter distances marked on it, then could you not print out a circular paper disc, with foot distance marks, to paste onto the dial? If you're really industrious, you could do hard conversions (such as interpolate between 3 m and 5 m ticks to find the right places to put 12 ft and 15 ft ticks) rather than less useful soft conversions (simply putting a 9.8 ft tick at 3 m and a 16.4 ft tick at 5m). The interpolations and drawing this disc on your computer would be a difficult one-time task but make the RF much more user-friendly. In fact you could put the foot ticks corresponding to the markings on the Bessa's particular lens, which would not necessarily be the case if you buy a foot-calibrated RF.