I've been investigating this too - I've found that when you look at the specifications of the tyres they usually vary a little from the results you get from the formula.
So use the formula as a guide - but get the specs on the tyres as a double-check (especially if the clearance is small) before parting with any $$$.
I noticed this difference when looking at the BF Goodrich web-site.
I found Toyo, Yokohama, Pirelli and Goodrich web-sites pretty helpful.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.