hi - i use an sl66 , there is a chart on the left side of the extension arm which tells you what the lens extenion factor is at the distince you are focusing at - the one with the 80> refers to using the standard planar mounted normally, the 80< refers to the retro mounted lens. There are also scales for the 150mm and 120mm lenses there too. The applicable extension factor is the small figure underlined by a thick white lineclosest to the camera body.

On the few occasions I have retro-mounted the lens, I have taken a reading from a grey card then applied the bellows extension from the cameras scale with no difficulties.

Hope that helps?? If not, there is an online manual here http://www.sl66.com/manual/sl66/sl66.htm

cheers!

Leon