Same as Dutch except for Mk1 Spits, I T/O with half Rad and leave it alone at around or just over 3/4 throttle and climb at around 1800 ft per min - same for level flight. Combat/full throttle - open Rad accordingly.

I've noticed that if I line up the top of the handle with the bulkhead that extends down from the dash board I can run at full power with out over heating.
I close it more in a dive with power off but There is no reason to open it more than this really.
Maybe if I was at high altitude and low airspeed climbing full power and fine pitch...

I have never tested it so see how much drag it really causes but I have heard the real pilots used to mark a line on the wall .