I'm not going to bother responding to this because as Dandywarhol has said in the other post, every vehicle is different. Also, you set the alarm to go off at a point on the scale that suits you which will also effect the setting. So whatever anyone posts on here it's not going to help you.
For a bit of help in how to set your own when you get it, on a long run, up steep hills on a hot day you can expect the needle to creep up towards the red on the gauge. Somewhere before this point, say about 2 o' clock set your alarm to go off. Just keep nudging the adjuster up as it goes off.
I had one of these alarms on my Townace which suffer from overheating far worse than the Bongo. I had mine set to go off early which unfortunately means the beeper goes off a lot but having had the head go previously it gave me piece of mind but annoys your passengers no end
It's a trial and error exercise and a balance between peace of mind and 'comfort'.