George, according to the formula for calculating offset (7"b.s - 5.5" f.s.)/2 = .75".
.75" x 2.54cm/1" =1.905cm or 19.05mm.
These wheels should have a offset of ~19.05mm. That is the same calculation that you arrived at.
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.