so the next estimated order date is calculated and is not a field within a table!!!!
If it is done this way, then what i would do is create a query for your first report and then you second report that you want to show orders that should be due in the next x amount of days.
In a query you can use the calculation that you have already writen.
So in a column you would have this
Next Estimated Order Date

ateDiff("d",Min([Date_ordered]),Max([date_ordered]))/Count([itemnumber])+Max([date_ordered])
this will create a seperate column for this field.
Then do for your second report, copy the first query, then just add <Date()-30 into the criteria under the Next Estimated Order Date.
If you are still confused it will probably be best to send me your mdb file, if that is ok with you.
The only issue is that you are not allowed to put email address into the post, so to get around this use this
kevin dot redfern at gmail dot com (hope that makes sense)