Counting until specific interval

Hi everyone

I am relatively new to this forum but I was wondering if anyone could help me with the following problem. I am trying to create a basic formula that will allow me to tell me how long until a set interval.

The basis of the problem is that I have a fuel bowser that holds 2000l and a counter that counts upwards every time fuel is used. The problem is that I want a formula that will tell me how long I have until an interval of 2000 is reached so I know how much fuel is left in the tank so that I know based on my reorder point when to order fuel. The formula needs to do this and reset once the interval of 2000 is reached. Any help would be greatly appreciated.

Kind Regards