Technique to avoid circular reference

I am trying to make a time based simulation for heat loss but I can explain my problem with a simple problem.

Lets say I have a bucket with water flowing in and water flowing out. If the bucket is full, the water flowing in automatically stops. The problem is that trying to put in an 'if statement' to stop the 'flow in' brings up a circular reference. Any ideas on how to get around this?

I have attached a simple spreadsheet to demonstrate.

I don't want to allow iterations because it really screws up how the program works.

Cheers