I don't think the distance down is too important. I would be happy to measure mine for anyone who asks. It's more than 4" I'm sure. Guessing 6-8" from top end of leg. I wanted to be able to spray lube right at the follower nut (the top of the inner leg where it is threaded), so that it could penetrate. Then run the leg down a little and spray some more. Then run the leg up/down to spread the lube along the leadscrew. You can't reach all the threads with just the 4" straw on a spray can, but working the leg up and down while spraying will get it effectively spread along the thread. To really do it right, pulling the legs would get it done. More work than its worth IMO, unless some other repair is needed.
Don't bother lubing the sides of the square tube legs. They are never a tight fit, unless you've bent one.