Just so happens I had to adjust mine today. Forget what I said about the 17mm wrench/socket. You don't need it. Just seems like most things on the truck require a 17 and a 19mm so I just always grab them.
I measured from lower a-arm mounting point. Not the a-arm, but right below where it mounts. That got me close to the point where the height would change the most. Technically you're supposed to measure the distance between the center of the a-arm bolt and (I think) the flat section of the bottom of the a-arm by the wheel. I'm sure Brent will correct me if I'm wrong. Mostly people use that measurement to get them within Nissan tolerances so they don't get dinged on warranty repairs.
Yes, bumpers can be misaligned. I've seen a lot of them. Also people often measure incorrectly from the fender well, which can be all over the place depending on if you turned your wheels, parked on an uneven surface, etc. That's because the shocks come into play so much there.