You don't say what kind of car you have. On my 240D there is mark on the shaft the holds the lid on the reservoir. It is filled nearly to the top of the reservoir when full. You need to buy yourself a Haynes manual for your car --- it will give you all those details in pictures (about $10 - $13).

Good luck!

__________________
1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold