Coldest night this year for the boiler. -10

I use a K-type probe inserted into the middle of the flue about 12-18" above the flue exit from the boiler. The probe provides output to a digital panel meter readout on my monitoring panel. Magnetic stick-on thermometers can be notoriously inaccurate. My general rule of thumb has been that they read about 50% of the interior center of flue temperature.