Definitely shouldn't be seeing that much error between set point and actual.

That's what I thought also, so I repeated test several different ways.
Always saw 40 to 50C difference.
Maybe it's because the Kapton-Film is still between bed and glass ?
Or maybe the hair-spray and air-bubbles, between them, is acting as an thermal-insulator ?
And there is a fan, used to cool stepper-motors, that blows some air over glass,
but not pointed directly at it .

Like I said , it's working Great with glass at 50C, so I'm not going to worry about it,
but Just seems Curious that about 100C is Recommended, when Not really Necessary or the best ??
(Since object warping is Not worse at the lower bed temp)

I don't have an IR-thermometer, but maybe I can borrow one.
Good Idea ! Thanks

Glass does not conduct heat well. IF you have any glass overhang, the glass plate is larger than the heater/build plate, then the heat loss will be greater. I am using thin copper sheeting now and it works as well as glass and much more heat is retained.

Now I'm wondering just how Hot the Bed really Needs to be ??
Since  only 50 C is working Great, with ABS, PLA, HIPS type filaments, that I've tried at that temp.
I'm starting to think the more commonly called-for 100C temp range, at object, is Too hot ?
(Of course the 'Set temp' still needs to be at 100C, for our printer with glass-bed)

Don't want to be Negative but It seems that copper would Warp much more than glass ??
It would have to be very Thick copper plate to not Warp
and then would be too thick to fit under extruders, at normal Z-zero position ??

I found that when Bed-plate temp is set to 100C,
the top of the 3/32" glass plate is only about 50C .
50 C seems to work Great tho, since had No problems with objects sticking to glass

BTW: Indicated Heat-block temps are very accurate,
and the fan added at X & Y stepper-motors reduced temp from about 70C to 40, on Y-motor