Pad material, caliper bore size, even tire sizing affects brake bias.

More aggressive pads will shift the bias in that direction. So, if you had organic pads all around and the backs were locking up you could change the front pads to semi metallic. Depending on what calipers you use there could be several levels of aggressive pads to chose from.

Brake pressure is measure in PSI, pounds per square inch. Caliper bore diameter will affect bias. smaller bore size means less pressure at the pad.

Tire size affects the bias, more contact patch means more braking. You may find them big meats in the back causing the rears to lock up prematurely.

RR, you are talking about a combination valve and it's hold off function which really comes into play in a panic type stop. Proportioning just affects pressure to the rear and at what pressure level it starts to affect it (hinge point).