Freezing calculated values as we move forward in time

I have a very simple model attached that describes how a column of numbers (Range A) is calculating the forward progress of a variable. The progression of this variable is only valid if the previous members of the variable progression remain unchanged. So, it's okay to calculate the latest member of the progression but not the values before it. I know this can be done with full VBA but I am looking for a user-defined function that might provide for a simplier solution. I have struggled for a few days on this and can't figure it out.
Thank you in advance for any offering of your time.

Steve Druley