The formula at the Landiss site is correct!
I just did a key-on test on my X11/3 to confirm the formula :
Battery voltage : 13.7V
Pin 3 voltage : 4V
Ratio : (4/13.7) x 100 = 29% (???)
If you use the formula :
[1 - (4/13.7)] x 100 = 70%
which is correct coz it's a federal car!
So the formula is correct!
Thanks for the heads-up, Mike.