Bruce,
Someone will correct me if I am wrong, but this is how I'd calculate this situation.

Take the I-20 at it's Portsmouth numbers in the various Beafort wind ranges and multiply that number by the SN modification factor for that same Beaufort wind range.

I come up with 60.38, 64.68, 61.71, 59.77 and 57.67.

This may be entirely wrong, but that is how I see the picture.

Hope this helps.

Dave