Hi Matt,
The preach is just a .005 per inch taper , starting it from .030 at the thin end to about .200 to the thick end , marking at .010 intervals. .010 - .020 -.030 etc.
Sure saves time on setting for thickness of lams.
I only use the preach to set the fence.
When getting ready to cut parallels, only check the fence to blade with preach, but when cutting tapers, I have a 1/4" thick(or so), for durability , taper in the right TPI (.001-.0015-.002 etc)that goes against the fence, then the preach you put the correct thickness at the butt of the thick taper.
So if you want a .002 taper, .100 at butt, just grab the thick taper, put the .140 mark on the butt end of it, and set the fence to that thickness.
Then use the thick taper against the fence when you cut the taper blanks, the xtra .040 is what you clean up the sanding marks from.