Scott, I'll see your .02cents--and raise you .02cents. I admire your quest for perfection (really-I'm not being facetious-I'm always amazed at the great wealth of knowledge here on the BB and the great lengths folks go to do it right) but the guestimate method works very well. If you actually measure the angle between fully extended and fully compressed you'll see there's very little difference. Unless the guess is grossly inaccurate through sheer incompetence it will be fine. There's always going to be some tension. After all the care you personally take--what happens when you plop down in the drivers seat? And even if you compensate for that - what about the guy you car pool with or when your in-laws are riding in the back seat? :-)
As I wrote--sometimes it just isn't practical to do it by the book and there are acceptable alternatives-- the OP needn't obsess over it. -- Dave