The length of a jumper isn't critical at all unless the amplifier's input circuit is maladjusted, or has no adjustment to it at all. If the transmitter's output impedance is the same as the amplifier's input impedance (or close), that jumper can be any length that's required to get from here to there. Sorry, there is no 'magic' length. Those 'magic lengths' come from not understanding what's going on. There certainly are specific instances where a particular length can be a benefit, but those situations/circumstances are few and far between. Using coax for impedance transformation is not the best idea in the world. It can be done, but always has a penalty.
- 'Doc