Jig transit, aligning ways

I bought a K&E jig transit with optical micrometer. From what I've
read they are supposed to be able to measure squareness and level to
a thousandth over a distance of 10's of feet. This sounded like just
what I need to set up rails on a CNC router. It spans a longer
distance than my master level.

The problem is that I'm not sure how to use it. I've used survey
transits but this seems a little different. Does anyone know of
online instructions on how to use one of these? Tried Google with no
success.

Roger S

PS. There are several for sale on Ebay, saw one for $99 without
micrometer. Mine is from LTV, seller has a couple more for little
less than $300.