That sounds about right. Bear in mind that all of the belts were slightly different in their construction, so there's no set way of doing it.
Here's a shot of the front of the belt that I made (minus the grommets):
The adjustable strap on my belt is sewn 3.75" onto the 3.0" webbing, extending about five inches past the 3.0" webbing to the male buckle (of course, this is on my tiny 30" waist). Overall, the adjustable strap is 20" long (the rest just gets folded under the belt and secured by the Lifegard).
Cutting about four inches off of the 3.0" webbing sounds right; it just depends on your waist size and how much of the thinner seat belt webbing you want showing. For example, Harold Ramis' belt (as seen on the Rolling Stone cover) didn't have much showing, whereas Dan Aykroyd's did. Personally, I like the two-tone texture and size difference of having the smaller seatbelt webbing.
Remember to use a lighter to melt the ends of both the nylon and the seatbelt webbing as to prevent the edges from fraying.
Hope that helps!