can't do it.
The most a millivolt reading will tell you is that the engine is running.
You would gain more data monitoring each cylinder exhaust port temperature.


Note:
Glow plugs are not thermocouples.
Tip temperature of the glow plug is measured using a K-type thermocouple


Basic uncontrolled variables:
* Electrode stress/torque damage
* Coke load in each prechamber
* Carbon load on each electrode tip
* Injector spray pattern
* Glow plug tip degradation pitting/burn off
* Cylinder compression variation
* Glow plug age degradation
* Glow plug brand/manufacture variation
* Injection pump issues/variable
* Fuel addatives, summer/winter variation


Any of these can cause false data.
A combination of these will cause false data in perfect LAB conditions.

A glow plug instrumented with K-type thermocouple runs roughly $500.00 - $7000.00 each.