Hi Sam,
Sorry for the delayed response but I've been somewhat preoccupied with a troublesome PC, which is still locks up occasionally, even after a repair install of Windows 7, along with countless updates etc..
Fortunately, I've been able to control my frustration, and my PC, Keyboard, mouse, monitors and printer have escaped unscathed.
Anyway, the PAS ratio basically affects how much motor power is delivered relative to the pedal rpm
(cadence), but I found it very difficult to set this correctly, and would often end up with too much assistance when I didn't want/need it or too little assistance when I did. I found it impossible to find one setting that suited all riding situations. Without the BAC-601 Smart display
(or Bluetooth on the MP5 etc.), there is no easy way of even turning it off when it wasn't required.
Unfortunately, using the pedal rpm to control the motor power is far from ideal, as when your pedal cadence slows down on a steep hill, you also lose motor assistance right when you need it most.
Torque sensing is obviously the best way of controlling PAS, but installing a pedal torque sensor is not cheap.
I have stopped using the PAS on my "Wheelie Machine" with the MP4 because I found it dangerous to use:
If I accelerated away from a standstill by pedalling faster in a low gear with the BAC-601 set to mode 3 or above I would often struggle to keep the front wheel on the ground.
Unfortunately, this was made even worse by the noticeable delay between when you stop pedalling and when the motor power completely stops, which meant that I always had to ready to quickly apply the brakes in order to cut the power and prevent the front wheel from lifting.
Even if I stopped pedalling as soon as the front wheel started to lift, the motor would still continue trying to flip the bike over backwards
(which is very easy to do on this bike) especially if riding uphill.
When I ride this bike now, I tend to use pedal power only, or pedal power combined with the thumb throttle to properly regulate the required level of additional assistance on hills, as I find the PAS to be unsafe to use in many circumstances. I could use it while riding at a constant high speed by adjusting the mode setting to achieve the required assistance and just changing down a gear and pedalling faster for more assistance, but I don't do many long rides at high speed as I mainly ride off road at slower speeds while "walking" the dog.
Alan 
