Matt,

Assuming that you are converting from bottom to front servo, here is what I did.

Route the old, too short, mixture cable where it needs to go for the new location and just measure what additional length you need.  Then pull out the old complete assembly and measure it while adding in the new measurement.  Round up to the next standard size and order it from Aircraft Spruce.  Since mine needed the solid wire end rather than the 10-32 threaded style, you install the new one and cut off the excess.  If you must have the 10-32 threaded style, the same method works, just be sure you have the bracketry all just right before you make the "additional length measurement" because then you will be ordering an exact length.   Another option with 10-32 threaded is the threaded extension that Spruce sells.  Just be sure that the extension does not make the cable action sloppy since there will be a longer distance from the last clamp to the servo lever.

If the above is not the conversion you are doing, then I think you could do the same thing with the existing throttle cable as the guide by temporarily removing it from the current location to do the fit and measurement for mitxture.

Tom Thibault