I believe when they mean back focus, it is the FULL travel of the drawtube.
Metal back is in reference for optical train adjustment items like reducers, flatteners etc. So, yes, the PRIME position for the sensor with this corrector will be 115.5mm back from the rear most metal face of the corrector unit. Remember, you need to subtract your sensor spacing from the front face plate (whichever this is on your CCD/DSLR) from this distance. So, say my CCD sensor was 15mm behind the front plate (not the glass window, but the front flat face), the adaptor from the rear of the corrector needs to be 100.5mm in length.
Capisce?
