Once you know the pitch of the Leadscrew, (Rotate 10 turns and measure how far the Saddle moves ) should enable you to calculate the pitch of the Leadscrew.
Knowing this it should be fairly easy to work out what ratio is needed to cut a particular pitch.
You will find the contents of one or both very useful reading:
Workshop Practice Series No 3, "Screwcutting in the Lathe" by Martin Cleeve.
Crowood MetawWorking Guides "Gearing of Lathes for Screwcutting" by Brian Wood
Having said that, with a small lathe it is likely that many of the threads can be produced using Taps and Dies.
Picking numbers out of the air, for some examples, it will go something like this.
Leadscrew pitch = 1.5 mm
Required thread is 1 mm.
So Leadscrew needs to rotate at 2/3 the speed of the work in the chuck
Possible changewheel set up could be 20 Driver, Idler 60, Leadscrew 30
Could equally well be Driver 50, Idler 20, Leadscrew 75, as long as the gears all mesh correctly.
Required pitch 0.75 mm
Leadscrew needs to rotate at 1/2 of the speed of the workpiece.
so, possible set up
Driver 30, Idler 50, Leadscew 60
or Driver 20, Idler 50, Leadscrew 40
If you want a thread of the opposite hand, just insert an additional idler. Since it will not affect the overall ratio, anything that fits will fit in, say a 20 or 35.
A 63 changewheel would enable you to cut Imperial threads (With some slight, but not necessarily ruinous inaccuracies ) on a lathe with a Metric Leadscrew.
If you just want a fine power feed, how about setting up:
Driver 20, Idler 100 / 30, Leadscrew 75. That should provide a feed of 0.12 mm per rev (0.00472/rev in old money 
? Hopefully, the stud and key will be long enough to allow you to compound the 100 and 30 wheelswith a pacer to put the Leadscrew gear into correct mesh with the compound Idler.
For setting backlash, (To avoid noise and wear, no geartrain should devoid of backlash ) just run a piece of paper (About 0.75 mm ) between the gears. Start with positioning the stud to set the Leadscrew / Idler mesh and then swing the banjo to set the Idler / Driver mesh.
HTH
Howard