I took a car can log specifically to find something we could use to give better feedback when you start to engage friction brakes. I think I found three signals indicated in the service manual. Target braking force (1CB:D3<<3+D4>>5), Regen Braking Force (1CB:D1<<3+D4>>5), and Brake Pressure (1CA:D1). Here is a graph of a simple test where I pressed the brake pedal three times at the beginning while I was at a standstill, accellerated, then slowly increase brake pressure until I stopped. I repeated again but more gently.
What is interesting is when you start to engage friction braking, regen is turned off
. They do not simply add friction on top of regen. I guess they couldn't get the feel smooth enough. So it tells us it is VERY important to avoid friction braking because you waste *everything* once you go there - not just an incremental amount above the regen capability.