SQL problem...

I have lots of values in a table. In that table I wanna create a query as shows a average value.
I created a Autonumber-field wich I divided with the field that I want to get the average value from.
But now it doesen't work, because sometimes you have to delete a row in that table.
And then the Autonumber-field doesen't work as it should for me.
I do like this.

VB Code:

SQL = "SELECT [Table1]![Invoice], [Table1]![Invoice_ACC] / [Table1]![ID] AS Average FROM Table1"

Does anyone have any suggestion how I should do?

Also I wanna divide the value in a field with the number of rows that are in front of the row.

Does this work?

SQL = "SELECT Avg(Table1!Invoice), Avg(Table1!Invoice_ACC) AS Average FROM Table1"