Excel Circular Reference Problems

Hi, I have two columns in excel like this:

A B
1 0.5
1.5 0.5
2 0.5
2.5 0.25
2.75 0.25
3.00 0.25

Basically the formula for A is the previous number in the A column + the previous number in the B column at the moment. So taking the first example, A1+B1 = A2 =>1 + 0.5 = 1.5 which is the next number in the A column. Now the values of B are controlled as follows: if 2.3<A, then B = 0.5, and if A>2.3 B = 0.25. The problem is that when the change happens, I want where A = 2.5 to actually be 2.25, not 2.5. How can I do this? If I do something like A2 = A1+B2 I get a circular reference... I don't mind adding other columns to do it, or anything that is required. Please let me know!

Basically my problem is also complicated because there are further changes letter on as A grows, so everytime a change happens there's a problem