Consider that where you install it (back of head?) might not be same temp as the thermostat.
I don't see the 5* difference as a problem?

All sorts of good quality switches and controllers here
http://www.spalusa.com/store/main.aspx?p=CategoryBody&c=FAAC

The "Automatic Programmable Fan Controller" from SPAL is what I'm expecting to use in my truck.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-