readings also depend a lot on the location of the sender...I find that fitting it under the plug is a PITA so I just put it on an exhaust stud
I put mine under a second nut on a head stud. Not as accurate as under a plug, but as long as you are just looking for a change, not an actual reading it's ok.
On a couple of circuit race cars I've seen the guages mounted at all sorts of angles and when I asked why, I was told that under normal race conditions, all the needle are pointing straight up - that way the driver notices quicker if something is wrong as the needle will move left or right and the thinking time for working out how different it is to normal is not needed!