When I took my GLC250, the SA from HSS told me the partial is 30% when the speed is above 60km/h.  So far, I never have the opportunity to experience this autonomous partial brake. Most of the time the Collision Prevent Assist triggers the alarm and I naturally step on the brake.

Actually, if human continue to drive cars (and not robots), better not to have 100%, it happened to me at NS highway, after a turn near Proton City, there was a sudden slow traffic. I stepped on brake and noted the car behind me was still speeding. I quickly swung to left as the car still had momentum. At the end, 5 cars piled up and I was excluded from the sandwich. Machines, at this moment, can't predict how crazy other drivers are.