rockyrude,
I don't recall whether you have a 6", 10" or 12" lathe but if being able to cut threads is important to you (it doesn't necessarily follow that it is), you should first concentrate on acquiring the additional dozen or so gears called for by your standard threading chart before you worry about whichever transposing pair of gears you need to cut the most metric threads. And get used to the fact that given an inch lead screw, all metric threads are going to be a compromise. Or another way of looking at it is that although theoretically possible, in practice you cannot make an accurate metric lead screw on a machine with an inch leadscrew or vice-versa because you don't have enough space on the banjo for the number of inch-based gears required. Or vice versa.