Need help creating forecasts informed by previous performance
 

Greetings! Thanks in advance for your counsel.

I work at a book publisher. We have lifetime sales data for over 200 books
we've published previously. When I look at a particular new book that has
been out, say, 6 months, I want to predict what that book will do in the next
6 months.

I've been learning about the Series, Forecast, Growth, LINEST, and Trend
commands in Excel. They're very useful for predicting the future based on
that particular book's past performance over the six months it's been
released.

However, I want the forecasts to also be informed by how those other 200
books have done in their NEXT 6 months (months 7-12 after release).

I wonder if I should somehow be using those numbers (rather, the average
and/or median of those numbers) as the "known x" numbers in these formulas.
Right now I'm using month numbers (1-6) as the known x values. Should I
instead be using, say, the median numbers for months 7-12 as known x?

Or maybe that's the wrong path entirely. Any ideas?

Thank you!

Jeff