Results w/ magnetic thermometers are usually a crude approximation...especially w/ a flat magnet on a round pipe. I do have one magnet type I use on a flat location and I calibrate it w/ a infra-red laser type, I pick a spot and shoot it w/ laser, then adjust the needle after the magnetic one has stabilized...you wouldn't believe how much it was off. My adjustable magnetic is an older model...newer ones may not be able to be adjusted.
A inserted bi-metallic probe type is hands down the best way to go, if it is an accurate one and you have a hole to insert it in.
Another tidbit...At the same internal flue gas temperature, a black pipe's surface is less hot than a silver pipe's surface. Anything black radiates much better than the silver.