The double contact sender unit opens at about 200 degrees.
I know this is correct because I just installed brand new OEM temp sender units and then checked them with a laser temp gun and a continuity tester.
If you touch one probe of the continuity tester to one of the temp sender terminals and the other probe to the other temp sender terminal the continuity sender will "beep" indicating that the internal points of the temp sender are closed. When the temp sender internal points open there will not be a "beep" indicating the internal points have opened. At that point the King Seeley gauge needle goes to Hot.
Go to
www.fordbarn.com and do a search on Temperature sending unit. Tons of info.