I've got one that goes back to 1997 - but it does not project forward at all. If I could project forward with certainty I wouldn't need to bother with the spreadsheet.
I project forward all my expenses at 5% p.a and then in the upcoming year I am more precise about costs so the 5% forward will change (generally down)
I assume any savings (that I don't drawdown) grow at 0% (most of it in equities).
It's a conservative approach (I hope) and at least it shows me I'm OK and allows me to 'indulge' perhaps more than I otherwise would