Greetings Mammoth!
I find, and this is just my own experience, that there are no hard and fast rules (or even a general rule) that determine the amount of th. I believe it's a combination of factors, and imo, the main characters in th are the pg/vg ratio and flavors, and nic level plays a small role.
I'm a 45-yr smoker but I don't like th...the softer the better for me. That's why I vape juices as close to 100% vg as possible. Prior to this month (have mixed all my juices down to 18mg for this month), all my juices were 24-25mg nic and had soft th (some, like Cignot's 25mg Chill has no th at all).
I also have a bottle of 36mg w/ a VERY soft th. Early on I had a sample of 18mg juice that had me coughing on every inhale...tried a friend's 12mg that was too uncomfortable for me. So if nic is the main factor, why did my 36mg have waay less th than my friend's 12mg? Her juice was 80/20 pg/vg.
You can also get strong th on certain flavors at 0 nic, e.g., members have said TV's AC has quite a kick at 0 nic. At the same nic level, my spicier flavors have noticeably more th than the mild flavors.
Again, unless someone has a rational explanation for the above, I'll hold to my original statement that th is determined by a combination of factors, but the main characters are the pg/vg ratio and flavors, and nic level plays a small role.