The general concept is that the CLB rating selected is tied to the takeoff thrust, the lower the takeoff thrust, the lower the CLB thrust. For the majority of airports which we operate to it really doesnt matter that it takes a few more minutes to get to cruise altitude.
We operate with fixed climb derates depending on the aircraft type, for some Boeings we use 10% and 20%. The climb derate is selected by the FMC in conjunction with the Thrust Rating used and the delta difference between ambient temperature and assumed temperature.
On older Boeings we used to use a climb rate of 500fpm as the change over point from CLB2/CLB1/CLB. On newer FMC equipped aircraft, the CLB derates will wash out around 14000 feet.
On another US product we are in the process of selecting climb derates manually, the policy will go something like, "when using DER1 and temp delta is above 10C, use CLB2. With a temperature delta between 0-10C, use CLB1. When using Max Takeoff Power, use CLB".
This is a general concept which works for the majority of our airports. For some such as GVA heading southeast on the SID, we can be better off using max climb due to distant height requirements on the SID, therefore avoiding a quick hold after takeoff.
You are correct in saying that the screen heights are 35/15 feet.
Mutt.