I’m not sure what proportioning valve you are using now or the one you are changing to. However, the Tilton valve I use has a automatic diversion circuit for threshold braking. Essentially what happens is the valve allows pressure to the rears under normal braking. During threshold braking a larger portion of that pressure is diverted to the front.
I thought that is essentially how the adjustable valves worked... taking a chart from the model Mark is using...
Brake pressure is equally distributed in the system up to a point, the "knee" if you will, and then outlet pressure only increases a small percent over inlet pressure past this "knee".
I first grabbed this Summit special one...
Free Shipping - Summit Racing™ Adjustable Brake Proportioning Valves with qualifying orders of $99. Shop Brake Proportioning Valves and Distribution Blocks at Summit Racing.
And now have this one on the way...
Free Shipping - Wilwood Brake Proportioning Valves with qualifying orders of $99. Shop Brake Proportioning Valves and Distribution Blocks at Summit Racing.
I had no idea how much pressure reduction I would need to dial it in, but apparently its more than the 37% bias the Summit one allows. That Tilton one looks like it can reduce bias by ~58% of total pressure, just like the Wilwood one. That Summit one would only reduce pressure to half way between position 3 and 4 on the Tilton model, I need those few extra positions to dial mine in.