circular reference problem

I have the following problem:
B3 is an input number that changes. B4 is a fixed number that is subtracted from B3. B5 is the result. B6 is the lowest possible number that occurs over a period of time. Obviously when I try to compare B5 to B6 and then change B6 if it is greater than B5 I get a circular reference notice.
This seems like a very simple problem yet I have been unable to resolve the issue. Any help is much appreciated.

Try enabling iterative calculation as a start,

File > Options > Formulas ... tick "enable iterative calculation"

I have tried that and I can't get it to work