I don't think I understand how one can use a caliper or sliding digital
micrometer for regulating valves.  I have both of those and have found
many uses for them but this is not one of them.

What Duaine was looking for is called a depth gauge micrometer.  It is
a round dial (anchor on the back) with a stem sticking out the bottom.
Moving that stem up and down is what registers on the dial in thousandths
of an inch.  I use it with a block or a device to hold it up in the air
above the valves.  I regulate the valves by blowing up the pouch and
measuring how much the valve actually travels.  It is also used on a
machinists lathe to see if something is turned true.

I think if Duaine's cheapo from Harbor freight has a light enough spring
return, it should work just fine for years and years of player rebuilds.
A more expensive one should not be necessary until el cheapo wears out.

D.L. Bullock    St. Louis
www.thepianoworld.com