Sales forecast report

How do you all make your Sales forecast report?

I Have on VB form field in Customer tAble for Slaes target but Slaes
target changes for every month for customers.

to use that field in Sales forecast we have to change the Slaes
target but then we cannot go back and see the sales forecast for
previous month.

Is it possible to make a database and join to vantge for Sales target.