WARNING: This page is about a different car, the 2009 Mercedes-Benz CLK350 and 2008 Mercedes-Benz CLK350. However, it is still accessible from the selected car via links, so may be relevant.
The coolant level must reach on a cold engine up to the marking (arrow) on the coolant expansion reservoir.
When coolant is warm: approx. 1 cm above this scribe mark.