Hi Mark, I believe there is an error in your code. When looping and using y = a+bx to forecast yi you are using x(i-1) then adding 1 (i.e. your parenthesis are off) instead of using xi:

MAX(A) + (MAX(B) * MAX(Forecastkey) + 1), -- Trendline
   (MAX(A) + (MAX(B) * MAX(Forecastkey) + 1))*
   (SELECT ...

Should be:
MAX(A) + (MAX(B) * (MAX(Forecastkey) + 1)), -- Trendline
   (MAX(A) + (MAX(B) * (MAX(Forecastkey) + 1)))*
   (SELECT ...

Outside of this, thanks for the very useful code! Feel free to contact me direct if you'd like to discuss.

James