The formula is in the Ford Racing parts catalog. You will need the tire diameter that you can get off the tire manufacturers website or Tire Rack.
I have found that the number of teeth listed may not be right. The changes may be due to the incorrect number of teeth on the driven gear in the trans. The Ford book has the typical teeth of this trans, and I think it is 6. Hee is the link to the catalog. The formula and parts are on page 153 (as printed on page, not the PDF page #)