Proportioning valve for rear discs

No...
A 4 wheel disc brake system does NOT use a proportioning valve unless the bias is out of whack. They can use a distribution block that has no proportioning to it. Those are found in 4 wheel drum brake cars where they were designed with specific wheel cylinder sizes front vs rear to achieve natural proportioning. Again, 4 wheel drum as well as 4 wheel disc cars do not need "proportioning" if the system is in balance.
2 to 1. That is the bias that is ideal. You want twice the braking force at the front compared to what you have at the rear. For example if the rear caliper piston is a single 1.5" unit, the front needs to be double the surface area. To make the math easier, you'd need a 2 piston front caliper with dual 1.5" pistons. Doing it backwards, find out what the surface area is of your stock front brakes and choose a rear brake kit with a caliper piston size that is within the 2 to 1 ratio.
If you have a situation where the front to rear bias is not close to 2 to 1, the crutch is an adjustable proportioning valve. A fixed/NON adjustable valve will not work. Traditionally, "proportioning" valves were used in front disc/rear drum systems to reduce pressure to the rear since the drum brake system has the self energizing feature and also because the fluid/pressure demands of the front calipers are far greater than the rear drum wheel cylinders.
A front disc/rear drum system without a proportioning valve would always result in rear wheel lockup.
A four wheel disc setup with a non adjustable proportioning valve results in the rear brakes never getting enough pressure.