There are a lot factors that must come together to have an accurate TC reading on a mod. A mere couple hundredths of an ohm can make a big difference with some of the wire being used.
The manufacturer of the board
Top tier, the
Evolv DNA line (made in the USA), but there are other good boards like
Yihi,
FSK,
Dicodes, etc
2nd tier boards generally wont be as accurate, and wont have as many options to tweak. Generally, if the Mod is $50 or less it is probably a 2nd tier board inside.
2nd tier boards can often be improved by running the
Arctic Fox software and Firmware and Tubo Evic which adds functionality very similar to
Evolv's Escribe software.
How well the mod was constructed, specifically with respect to the solder joints to the 510 connector, and the solder joints to "ground".
A cold or poor solder joint can increase resistance which will throw your measured temp off.
Is it a quality 510 connector, many spring loaded connectors dont have enough tension
Was the mod calibrated to account for internal resistance Better boards will allow you to plug in an "offset" resistance to compensate for the accumulated resistance of the the 510 and internal solder joints
The accuracy of the TCR or TFR curve being used with respect to the material used in coil.
The vast majority of TC mods (FSK being the exception) rely on "knowing" what resistance to expect at a certain temperature. This relationship is expressed as either TCR or a TFR.
TCR (Temperature Coefficient of Resistance) which is a single number that the TC Mod will use in its algorithms to calculate the temperature.
TFR (Temperature Factors of Resistance) which is a series of numbers representing a curve of numbers that the TC Mod will use in its algorithms to calculate the temperature. IMHO the TFR is the more accurate of the two methods since the relationship is usually not linear.
The bottom line is that ANYTHING that might increase the resistance in your mod/atty/coil will cause your temperature reading to be inaccurate.
Some tips
Search out your mod info. Different mods use TCR values differently it seems.
https://www.e-cigarette-forum.com/t...tcr-tfr-for-all-the-different-tc-mods.742439/
Most TC mods will allow you to set a maximum wattage, set this wattage to be about 25% higher than the wattage you vape at. This is only necessary in the beginning as you learn TC. Once you have learned it, and are comfortable with your Mod being accurate, then you can adjust your wattage, as the TC (technically temperature "limiting") will automatically limit the wattage to keep you from going over-temp
Dont take a "hot" atty off of another mod and then put it on a TC mod, the results will not be good. A TC mod measures (what it assumes is) the "cold resistance" the first time you attach it to use as a baseline. If you attach a HOT atty your baseline will be off and your temperature readings will not be accurate.
More advanced reading on this subject
https://www.ecigssa.co.za/guide-to-fine-tuning-temp-control-vaping.t18206/
It is wonder noone recommends the vapcap as a better alternative to the stempod.. 
I love the threads where someone is looking for an electric portable or desktop and he gets recommended a butane powered vape because it is so damn sexy,rofl.. 
) claptons I made by wrapping round a trimmed down lollipop stick that comes out at .16ohms which holds temp effortlessly for a quick extraction or a longer drawn out cruise session. 
My mod isn't able to run FJ firmware so not sure what are best settings. Dan includes TCR in the stempod manual. It be the correct one for included coils.
