Most guys typically measure directly in front of a tire unless there is some big reason to measure elsewhere. The reason for that is that the first thing you have to do to get up an obstacle is to get a tire on it. Large steps are often impossible straight on (how do you climb a wall that is taller than the truck?) so the driver usually goes at an angle, getting one front tire on the obstacle, then works the rest of the vehicle into place. Once a tire is up, the approach angle is a non-issue for the most part (but the departure angle may come into play).
Here is a picture describing what I just said. Tires are on the obstacle, but check the departure angle, which gave me fits until I cleared the bottom of the hill.
Just for fun, check out the tire marks to the right and above my 'Sploder.
Take some building to get a Trailblazer up that -- that IS the goal! Who's gonna be first?
http://www.UCORA.org == The United Christian Off-Road Alliance, a family friendly place
2003 Trailblazer = got groceries?
1995 Dodge RAM 2500 w/Cummins = got tow rig?
1986-90 Ford Ranger truggy = got rock?