You may wish to search for a product call Plastiguage. I purchase it
at automobile supply stores.
Its intended use is to gauge clearances on crankshaft bearings. It
may be used to adjust the parallelism on a rolling mill rolls; place
two strips on outside edges of the mill rolls around the
circumference of the rolls and roll through.
The tighter side of the mill will squish the gauge strip flatter and
wider than the loose side. Adjust the loose side of the mill tighter
and retest(this may require the application of additional bits of
plastiguage). Continue to do this until two strips of plastiguage on
on opposite sides of the mill during a single pass are the same
width or as close to the same width as you can get them.
Reinstall the adjustment key/gear at the top of the mill.
There is one other thing to consider. If you own one of the newer
low cost rolling mills which are offered with
multiple rolls (flat, wire,textured etcetera) you may need to check
the tightness of the bolt together mill frame.
If parallelism problems continue I also might suspect that (even with
tight bolts)small errors in machining (allowing the
Mill frame to flex or distort slightly during operation) may be
causing the problem.