The ultimate test is leave it ticking over for ages till the fan has cut in and out a few times, if it doesn't overheat don't worry.
I would as you thought assume the fan would come on for a minute or two, but if it's doing the job then don't worry.
I would say your guage is out, same as my car, my fan cuts in at 95C, then off at 92C, this can be anywhere on the gauge from the "M" to the "R", so I know I have a dodgy gauge!!
tabetha