A 650b tire is about 1" taller than the same size 26" tire. So 3/4" is plenty of room. However you need to take in to account the shape of the seatstay and chainstay. If the curve is too narrow the side knobs of certain tires may rub. On FS bikes you must also consider if it will rub the seatube or front derailleur at full compression.
2013 Santa Cruz TRc 650b
2015 Lynskey Cooper CX
2010 Soma Groove
1987 Haro RS1
what year and model bike are you trying to fit the 650b wheel on the back ? there are threads with bikes that will fit at the top of the 650b folder plus you can go to 650B Palace for a good list of bikes.