Your post seems to be asking excellent questions, basically, what constitutes "normal" with respect to operating temperature when you are not able to measure that blasted fluid to see whether it is at 80 degrees C, and then have you filled it to the correct level for whatever temperature it was at when you did the work?

I venture there may be no simple answer, except to suggest that your first advice to run it for about 20 minutes was the best advice. You see, the transmission is cooled by the radiator and once the car runs up to normal operating temperature and stabilizes there, the transmission oil will likewise have attained approximately the same normal temperature as the radiator, roughly 180 degrees F these days and which happens to be, coincidently, very close to 80 degrees C. Therefore, setting the fluid level to the proper mark after warming up as advised should be fine.

The transmission is designed to accomodate fluctuations in level due to temperature changes, but over-filling can be bad if the rotating internals foam the oil, the pump will pick up air bubbles rather than oil. Filling to the upper mark when cold will cause the transmission to be too full when hot; filling it to the lower mark when hot equates to under-filling which could lead to a dry pump suction if driving conditions stack up against you. This is also bad news. So, set the oil level as previously advised, look for tell-tale red spots beneath the car and change the oil often to keep the transmission healthy.

For what it's worth...

230/8