Add and Average customer's monthly data, then Display Top Ten Records

[ASavage]

We have three billing cycles each month (the dates vary from month to month), each comprised of approximately 5000 customers. What I need to do is get the annual average usage for each customer, then display the top ten customers by usage. I've struggled with trying a cross tab query as well as adding the usage from all 12 months for each customer and then averaging it, but don't know how to write it as the day varies from month to month. For instance, in January the cycles may fall on January 4, 22, and 28. Then February they could be February 5, 18, and 27.
Just to give a "for instance," my fields are: AcctNo, Customer, TransactionDate, Usage.
Each Customer will have 12 Usage(s) on 12 different TransactionDate(s) (one each month) for the year. I would like to average out their Usage for one year and choose the top ten Customers by usage. Hopefully I've explained myself clearly. Please ask questions if I've been unclear. Thanks to everyone in advance for any suggestions!

[ranman256]

I have a form with date range boxes,to set for a day,or month,or year range.
txtStartDate, txtEndDate.

Q1 pulls data in that range:
select * from table where [date] between forms!myForm!txtStartDate and forms!myForm!txtEndDate

Q2 calculates the Avg from Q1.
Q3 combines Q1 & Q2 on the ClientID

make reports from that.

[ASavage]

Hrm, ok, I've managed to get all 12 (or however many months that particular customer has been active in that year) of the monthly records for each customer with your suggestion, but then how can I group those 12 to average them in the 2nd query? (Sorry if it's an easy fix, but this really has me stumped.)