Temp fan probe location.

I have put the adjustable temp. fan switch probe in top of the radiator.
The cooling fan kicks inn way too early, so I have probably done a mistake?

It's not possible to remove the probe without destroying it.

I have a gauge in addition. With the sender located in one of the cylinder heads, and as far I know, the gauge works ok.

Engine is Chevy 350 in a 34 Ford Roadster. Afco aluminum radiator.

Suggestions?