David, I am not familiar with the CabMate preformer, but others I
have seen and used have a variable stop on them to adjust the size
of the stone. This is usually on the stop that the cam rides on.
By using this stop, you can preform numerous size stones with the
same cam. IE, if you use a 0-2 oval cam, you can cut a 3x5, 4x6,
5x7, 6x8 7x9, or 8x10 stone, just by varying the stop. In other
words, that cam will cut a stone 2mm longer than it is wide,
regardless of the dimensions of the stone. If you set the stop for
a 6.5 width, you will get an 8.5 length. This is how it works in
theory. With my hand crank preformer, I will usually end up with a
stone that is about 0.5mm short if I cut to the width. From your
description, the preformer has a motor drive and should give better
results. Look for the adjustment screw.