RB67 focusing with extention tubes, using a rail system

Greetings,
Perhaps I simply don't understand how, or an I simply am not that
bright, but it sure seems as though focusing when I have an extension
tube attached is a tad more difficult than when I don't have the
extension tube.

Which begs the following questions:
1) Is there some method to the madness. It always seems as though I am
just a little off from what I need to be.

2) Is there a rail system that will allow me to change the camera's
distance instead of the focusing mechanism? I have seen some outh
there but question whether they can handle the RB67's weight.


Thanks in advance,
Tom