Below is an excerpt from an email Paul (cellman) sent me about that very question. I'm now using an external shunt stand-alone CA and a wheel magnet with my Mac.
"The 12 fet should have a shunt resistance of approx 1mohm. The Mac has an effective pole count of 80 due to a 16 pole motor and a 5:1 gear reduction, but you should ideally use a wheel magnet when using a geared motor as it will not register distance correctly, when not under power, distance is not recorded."
MAC geared kit from Cellman, KHS softtail frame. Front 9c from Ebikekit, Jamis Commute frame.
Rear 5304 from Maxwell, fs MTB frame. E-Fada scooters x2.