I haven't tried all of these gear combinations, so they're
presented as a try it 'n see formula.

This gear chart/list was created by someone from a Yahoo
9x20 lathe group, a couple of years ago.
The lathe models include most of the imported 9x20 lathes 
with a quick change gearbox and half nuts for threading.

A quick method to check lathe gear selections is to mount
 a dial indicator to measure carriage travel, and count 
spindle rotations.

A mark on the chuck or faceplate gives a fairly accurate 
way to count revolutions. Take the slack out of the 
geartrain while zeroing this mark, and watch the DI for 
1.00" of travel while hand cranking the spindle the 
same number of turns as the threads per inch you want to 
cut.

Of course, if the count doesn't come out the same for 1" 
of travel, the correct gears aren't in place (or the 
quick change box ratio is incorrect).