For the 'what temp should I be seeing' question I'd say it very much depends on your own scooter, the set up & (mostly) where you've got the sensor.
For my own GT240 I've drilled & tapped the centre of the head (where the unfitted centre sparkplug gap is) so the sensor is very close to the heat source. Because of that I will usually get 220c+ but also get a near-instant reading on temp changes. For me, I want to see the temp dropping quickly if you lay off the throttle or reach the top of a hill, if it keeps rising or is unusually high I'd get worried.
Those red under-the-plug sensor holders are shite, I've found they disintegrate over time & then you're potentially left with a air leak under your plug, so much prefer it fitted to a fin or in the head, or something along those lines. It's an unusual 4.5mm thread if I remember correctly.
I found once fitted you get used to what temps are normal for you & will know when something feels wrong. It does give you something extra to worry about, but it's worth knowing