Re your focusing problems, what are your cameras? I ask because as I calculate it 18" in front of the lens is ~ 4.5 focal lengths, so on your 8x10 extension must be ~ 1.25 focal lengths, i.e., 160 mm, of which ~ 25 mm will be due to the shutter.
Don't take my estimate of the distance as too exact, but both cameras are Eastman 2-Ds, which have pretty hefty standards front and back. I have a 111mm lens for the 5x7 that *just* focuses at infinity with the standards together---but it's in a Rapax shutter and conventionally mounted, which accounts for the extra few mm. The Perigraphe doesn't look like it misses by much on that camera, but on the 8x10 it's much more drastic.
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_