I would think changing the tumbler gears is a hard way to achieve different gear ratios. Assuming you do not change the spindle gear then the only one of the tumbler gears that would effect the ratio is the one mounted on the axis of the tumbler assembly. To change this it would involve making a new tumbler assembly and moving the pivot point. I think just tying to work out a ratio using your existing change wheels is the first method to try. If you cannot achieve the pitch you require then you will have to obtain some extra change wheels. The way I do this it to use the program "nthreadp.exe" which you can download from here. Work out all the ratios given by the gearbox and multiply the actual pitch of your lead screw by these ratios to give effective lead screw pitches Try running nthreadp.exe for all the effective lead screw pitches you have calculated. You may get a result that is close enough to the value you require. To get exact metric pitches you will probably need a 127 tooth gear. (Assuming you can find room to fit it on the banjo.

Les.