I bought one of these and glued it to the garage floor so as soon as my tire crosses it I know I am in far enough.
http://www.amazon.com/D-2-Rubber-Duct-Cord-Cover/dp/B002C8CIVM
Installed my rails, about a 15min job and they are great. I like that the are powder coated, and well built. With my floor jack I can lift both wheels off the ground with 3 pumps. Should be everyone's first mod.