Not so much a matter of how much pressure, but the percentage of pressure that is going to the rear.... I usually start with about 40% on the rear, then tweak it in with the proportioning valve after that with a single master cylinder....... With dual master cylinders, start by checking the pressure at the master cylinders, then work from there with the bias adjustment to increase the rear pressure.... Are your rears the big or metric GM calipers??? The big one's also take a ton of fluid to work and depending on your pressure at the rear, could even need a bigger master cylinder.... I usually go with equal sized cylinders and work it out with the bias adjuster and pressure gauges....