How do I create a dynamic SUM function in Apple Numbers with the most recent value from column B?

In Apple Numbers I am trying to use a SUM function where I add one value in a set box (e.g. F7) with the most recent value input in the B column. Column B will have new values added monthly on the spreadsheet, and I want a function that will add F7 with the most recent input into column B, NOT the total of all values in column B.

Is there a way to get this to work without manually updating the SUM function each month when a new entry is put into column B?

[Re-Titled by Moderator]

Original Title: Apple Numbers