Normal operating temperature is, as you are probably aware, somewhere around 215-218*F. I'd look into having them come on around then - and you can use that as a baseline to experiment from (I've got my operating temperature pushed down to about 190* with the closed system, with no ill effects, and reduced tailpipe emissions! If they'd only let me do away with the EGR now...)
I realise that many OEM's are now designing the system so that the fans will run after the engine is shut off, but I see limited utility in this. After all, the coolant isn't circulating - so how much heat can you really get rid of? That, and the Derale fans are, as I recall, pretty big "current sinks" - don't they take something like 30A fuzes? Anyhow, I'd want a little extra reserve capacity if I had those fans running after the alternator wasn't.
If possible, you might want to "stagger" the fans - have one come on at 195-198*, and the other at 212-215* or so. This would make for more efficient cooling than having both on at a more elevated temperature. But, that's just my line of thinking...
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