Doesn't matter how long the hose is, just long enough to reach from the carb to your ear. In a pinch, you can just lean down and stick your ear next to the carb (tho it's much harder that way).
But I have to say that, for the SUs on a TR3, I much prefer the SU tool kit synchronizer; as it makes it easy to compare flow in real time, so you can see how far the linkage deflects between idle and off-idle. With car, you can get them both synchronized. Of course, that doesn't apply to Webb's DGVs.