I'm exactly your height and ride a 58 BN and it feels exactly right to me with a 100 or 110mm stem and straight seatpost. Seat tube center to center means from the center of the bottom bracket to the center of the top tube...thats a relevant measurement only if you have a horizontal top tube. Center to top is basically meaningless, because different frames have seat tubes that extend different lengths above the top tube. Forget about seat tube length and base your decision mostly by top tube length. This could be problematic too, because if your road bike has a sloping top tube, you have to go by theoretical horizontal tope tube length. Does your road bike feel too big? If not, I would stick with the size frame the most closely matches your road bike...that would be appear to be a 58 Bareknuckle. You could drop down to a 56 and be OK, but you would need to compensate by using a longer stem and run more headset spacers to get the same handlebar position.