RTH - does the temp gauge move around based on how "loaded" the car is. My '85 300D does. When I get in traffic it will get closer to the 100dC mark than the 80dC. Once on the road it drops closer to the 80 mark. In addition, I have not run it since yesterday afternoon and I am sure that there would be some pressure in the reservoir. The thing is, as long as it runs good - don't worry about it.
In the E320 I run "water-wetter" with approximately 40% MB coolant. I have yet to see it touch the 100 mark since going with this set up about a month ago. I can tell when the aux fans come on and go off by watching the temp gauge and considering the engine load.