[HELP] Can someone help me to re-gear my lathe?

[Dan Boy]

So I'm looking at gears and I'm just a little confused how to work out the length / size of the gear train. I know I have a fixed point on the driver gear and a fixed point on the driven gear. I'm happy to relocate the idler gear.

Is there a method of calculation, because I am aware that the gears will mesh together, so that I can come up with three diameters of gears that will match the current length of the train?