No, this won’t work – it’s not going to give you a monthly average, just the average value of invoices for the given customer.
To get monthly averages try something like:
Code:
Field – Formula (Currency)
Summary Type – SUM
Formula – {amount} / 12
Also keep in mind when going with this approach –
1) This won’t take Cash Sales or Credits into account
2) The results will only be correct for customers that have been with you for at least 12 months.
To test your search I would suggest choosing a (few) smaller customer(s) and manually calculating your expected results to make sure they match.