I left the bed heat up for a about 10 minutes  at what the thermistor reads as 65°C, then measured the center, the corners and a couple of spots around the middle.

Center        =  88°
Rear left     =  79°
Rear Right  =  82°
Front left    =  81°
Front Right =  84°
Other         =  85°-89°

So the thermistor readings seem to be off by about 20°C.

FWIW, the PCB bed heats up to temp in about 5 minutes, which is pretty nice.

Now the problem:  I don't have a IR thermometer handy, but I can tell the reported temps are NOT accurate.  The bed feels hotter at 60°C than what the old bed reported at 95°C.  I even lowered it to 45° and did a 3 hour long print and no problems, it held on to the print perfectly.

Doing a PID doesn't make sense, since I don't know what the actual temperature is.  So for the moment I'm using it at 50°C max.  But I'm wondering if there's something I did wrong when I mounted the thermistor.

The setup is:  The PCB is installed on top of the copper platform with VHB tape, which give about 1mm worth of clearance between the platform and the PCB.  The platform has a hole drilled in the middle through which the thermistor pokes through and it goes into the hole of the PCB.  Then there's a sheet of glass on top.  ID did not epoxy or otherwise permanently attach the thermistor to the PCB heater, it's only sitting there in the hole, so I wonder if that is the reason why I see the temperatures I see.

I'm thinking about using some thermal compound (like for CPUs), to pack in the PCB hole around the Thermistor.  Any thoughts on that?