Hi Tom:
If I am understanding you correctly with the right category in mind it would be a measurement in inches.
It appears that you can not use any of the standard products such as V-600 which requires 20 square inches per lineal foot net free vent area or others with varying square inches in NFVA.
Since you have to customize and so you don't create a voided warranty, I am referring you to Cor-A-Vent's site, the company has been around since 1976. Their detailed Q&A section should be of help. http://www.cor-a-vent.com/ventilation-faqs.cfm
You might gain piece of mind by contacting them to see if they can do the customizing - you'll get the help and know the sizing is right.
Jane Doe Chronicles
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&A should help you.