Basically the rule is 1000mL = 1L so to convert mL into litres you divide by 1000 and to convert litres into mL you multiply by a 1000.
So 1600mL in litres is 1600/1000 = 1.6L and 1.653L in mL is 1.653*1000 = 1653mL.
Basically with the notches you need to think how many notches are in one litre.
If we have say 10 notches in one litre then converting this to litres would be dividing by the number of notches.
So if we have a value of 165 notches where we have 10 notches per litre then the number of litres will be 165/10 = 16.5 litres.