Spitfire Rads - How closed do you go?

[PeterPanPan]

I fly mainly Spits with CEM on. I also always fly with rads fully open to avoid overheating. But I suspect this is being over cautious and slowing me down too with the drag from a fully open rad. So, the question is, how closed can you go in average flight conditions to get the best balance between speed and avoiding cooking your engine? I suspect there is no simple answer, but let's see ...

Thanks

PPP