Trying to Write a report to see how my sales are what tables do I need to look at?

I will be making a number of reports and though best to create some views in my database that have as much information as possible in order to supply the data to the myriad of reports I will write.

CURRENTLY I HAVE:

- OEINVH joined with OEINVD so I can pull all the items sold qty and price

- Then I subtract credits and add debits from OECRDH Joined with OECRDD

My thought was to check against the O/E Sales Statistics but the Net Invoice amount s off by around 1% for every period. 

Does anyone know from where O/E sales statistics Net Invoice Amount is gathered and how? I assume these are in statistics tables but the values should have come from entering invoices, credits and debits.