Buy a 25 dollar rinky dink meter - these are plenty good enough for our use .
Find a spot to check -that is easy to get at after flying.
I like to use the intersection of barrel and case fins on any engine .
adjust your engine to run correctly (not using the meter ) for your flying style -for some guys this will be richer on mix than for others -this is NO magic setting
If you plug is reading correctly and th engine is not carboning otr scoring and feel nice and slippery when bouncing th prop - you are likely in the right ballpark.
check the heat the same way -within a minute of same time etc., after a flight .
I would bet that you get a reading in the spot I mentioned around 200 degrees .
If you reading changes say 20 degrees for some reason - find out why and adjust it
Also - th meter will show big differences in temps on a twin - this can be a tip off to bad airflow.
All the meter is doing is helping you check and compare .
But once you learn to use it - and you must learn to use - it will help you keep your engine running correctly.
There is no easy answer to which measurement setup is best .
These do help -but you have to learn a technique which is easily repeatable .
If this were an EGT setup in a Lycoming- then yes you could simply ask "what is the correct operating temp for a given situation.
But these ain't Lycomings- and your setup is different from mine or likely your friend's setup.