I would go for .032" valve clearance or travel if the valves are
covered in leather that is also about .030"-ish and is not pouch
leather.  When valves are covered in pouch leather you can have as
little as .028" travel.  The idea is for the valves to not make
a hissing sound as they work.  If they hiss, or 'spit' as some say,
then the travel is too great.

I would also mention that when valves seat on brass that you use the
smooth side of the leather for the valve facing; when valves seat on
wood then use the suede side.  I would strongly recommend that the
brass seat be cleaned with a fine Scotch-Brite pad and sealed with a
lacquer made for metal to prevent a chemical reaction with the brass
and the tanning acid in the leather.  You might consider gluing the
valve face with glue just on the outside perimeter of the valve
allowing the stem when glued in place to have a slight wobble.

Finding just the right cow hide can be very tricky.   Sheep leather
will leak.  The thinner the leather, the more airtight.  My first
player was a Kimball with the Bakelite valves.  Use beeswax to seal
the top seat.  I am sure others may have a different opinion.

Don Teach
Shreveport, Louisiana