Adblue in general if a adblue reading is for example 1/2 full 2000 mls how does this reading drop is it only every 100 mls or so

[mercmancdi]

Asked this today and kinda stumped me. /::: Adblue in general if a adblue reading is for example reading 1/2 full 2000 mls left how does this reading drop is it only every 100 mls or so or should it go down by the mls ie do 30 mls should reading then be 1970 mls
or how does it work.

 

[CoopsSA]

My Volvo doesn't have a mileage specific adblue reading, it's just buried in the car status menu and states how full it is. 1/4, 1/2 etc. When it gets to a certain level it reminds me to refill - if I don't refill the car won't start!