I use a spreadsheet too. It's the only way I can be sure about future status point levels.
In Google Sheets, I essentially have column A (date), column B (Status Points for flight), and column C (rolling 12-month total). In row 10, for example, the formula for column C is:
=SUMIF(A1:A10,">" & EDATE(A10,-12),B:B10)
You then only need rows for your actual flight dates.
Of course, you also need other columns for SPs earned from NZ flights, SPs earned from non NZ flights, SPs earned from credit cards, and another formula for calculating the rollover of a status tier you've already reached, etc.