@boxcarman Not 1m, but 1.5-2m.
Start of transition at the source creates possible reflection at the end of the cable (slight impedance mismatch) that comes back and adds to the this edge changing its shape. In order to miss threshold point (middle of transition) total travel time (forth and back) has to be more than half of transition time. Typical transition is about 25ns. Half of it is 12.5ns equivelent to 2.5m total both ways resulting in min 1.25m one way (assuming 5ns/m propagation). Of course it is all guessing since propagation time vary (is dielectric dependent), while transition time can also vary. So this is wide guess, but it doesn't hurt to aim for longer cable.
