uh dah jee oh! I think the accent is on dah.
Heh, I mispronounced that for well over a year, uh day gee oh ...
The water should be realllllly boiling. Not hissing. Let it go to the full boil.
Also, if you need to calibrate, do it at freezing versus boiling since unless you are at sea level, the boiling point will be lower. And allowing some steam to escape through the spout will not cause a decrease in the water temp by anything appreciable.
If I pour boiling water into a cooling vessel, the temp will drop by 20-30 degrees fairly quickly.