If the kickdown rod is too short, you'll be in third by the time you hit 5mph (IOW, the shift points will be low) In that case, lengthen the KD rod.

If the KD rod is too long, the shifts will come at very high rpms, and may not downshift correctly when you slow down. In this case, shorten it.

Then the ratio of the lever on the transmission also comes into play, but I don't know as much about that.

-Dave