Top N using a formula

[Bennie47250]

Using Crystal version 7.0

The report groups the number of receipts and defects for each vendor for a given time period.

I have created a SUM on both the receipts and defects to total each. I then created a formula, that uses these sums, to determine the Parts Per Million of defects. The formula is named “YTD PPM” and the formula is :
Sum ({@Inspections}, {Inspection_and_Receipt_Records.VNDNO})/
Sum ({@Receipts}, {Inspection_and_Receipt_Records.VNDNO}) *
1000000

I would like to have the report run and only show the top 10 of the YTD PPM formula. However when I select the Top N /Sort Group Expert, the YTD PPM formula is not shown.

How can this be done?

Thanks
Bennie