i find just put the power where you want it for the best balance, and takeoff, and where the grip is generally needed in relation to front/back, as a start.
after that you figure the center diff wont do much on road as you should be keeping all 4 wheels on the ground most of the time, and as long as you have some locking front to back you are all set front/back wise.
for the front its all about driving style. if you tend to push the car hard into corners, and a locked diff will pull the car the way you want, then put a high locking in the front. i like using clutch, because you can control what the car is doing with your right foot, especially in a slide.
if you are more conservative, and use the "slow in fast out" technique, then best keep the locking relatively low, or even open so you dont understeer.
in between, just go inbetween those 2 extremes.
for the rear, i find it best for a all wheel drive car to have a lower then normal locking, but still enough to not lose power on a tight corner, and still give the slight controlled oversteer you need to stay in control and comfortable.
any other questions, or need clarifying?
the first part about the torque split, and the front/back torque was kinda just what i could quickly think, but the how to set the rest of the locking in the front and the back was well thought out, but still not be too correct, as i prefer xrt, and dont really ever race the fxr.