Hi Paul!
You're absolutely welcome!
It is strange the way it's done, bemused me when I first discovered it many moons ago!
On the later versions it is indeed far more difficult, involving reading fluid temperatures and an "overflow" fluid height tube in the bottom of the transmission pan, bit of an unnecessary change in my grumpy opinion!
But on a traditional one, way better!
The COLD level reads between 1 and 2 for min / max, and HOT is between 3 and 4 for min / max again.
Yep, I would agree with you that a 20 minute drive, at this time of year and ambient temperature, should have the fluid up to normal working temperature.
Cheers, Nev