TPMS - At what Temperature do I worry?

I have done a lot of digging on that same question and the answer eludes me also. But from my perusing I did find there is no magical number. It seems the most common answer is a tire that is not consistent with the others. For instance if all your tires are running 125 and you have one running 150 you probably have a problem with that tire. There are other factors you have to remember like the sunny side. If your running on the highway and the blazing hot sun is on your left or drivers side cooking the left side those tires will be somewhat hotter than the ones on the right side. How much?? Take a SWAG. From my experience with a digital infared thermometer about 20 degrees. So if my shaded tires are running 120 and my sunny side tires are 140 to me thats normal but if you have the same scenario and one shaded tire is 140 that tells me there may be a problem.
If any tire gets next to the 150 I would let them cool, but thats just me and not based on any scientific information. I think from experience that would be about my safe limits.