Which nest reading are you using? The one from the nest display itself, the website or anyone's Nest device other than mine?
All the nest devices here had a horrible truncation bug, they all converted a double value to an int before rounding.
So if the nest was reading 74.9 the device would truncate that to 74 when the nest website or actual device would read 75.
My version fixes that...
The same issue exists with the ST Multi temp reading and I fixed that as well in my version of the device.
I still wouldn't trust the ST Multi temp readings to run a tstat remotely from, the poll interval is fixed at 2 minutes. You don't want to be triggering your AC on and off every 2 minutes because you got one reading at 77 and then 73 the next 2 minutes.
In order to do it properly you need to add some smoothing or timer to run the ac until a setpoint as been reached.
Ultimately, the Nest really can't work off a remote sensor, because it will run to a temp and stop. You would have to overshoot the temp you want... For example you send the nest the command to cool to 73 degrees. That is the temp the nest will read.
So, if your remote sensor is reading 75 and you want it to cool to 73, you would need to get the current reading off the nest (not impossible) and do the math to set the nest to 71 instead. Then in theory, your remote sensor would eventually read 73 and the nest could read 71.
Unfortunately, temp sensors are not created equal, and turning on and off HVAC systems 15-50 times a day is not good for the motors, etc.