If you remove the cap on the expansion tank and look straight down you will see a plastic tab protruding from the corner of the tank. The correct level is when the coolant is at the top of that tab.

There should be an arrow pointing to a line with the word 'Kaltwasserstand' above it- this would be the cold water level in the expansion tank. On my cars this line is just above the seam between the upper and lower parts of the expansion tank.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)