Back in the early 70's I built a set of 4 mercury manometers to synch the carbs on my '70 CB750. I bent the glass capillary tubing into a 'u' shape with a propane torch and assuming the id of the tubing remains constant (and I do), there's no calibration required. Originally, it was recommended to use surgical tubing to connect to the carbs. But that stuff costs a lot and deteriorates quickly. It was used because it had good damping qualities. But I'm now using drip-watering tubing which is cheap but quite rigid and there's no nasty fluttering. I think this is because of the inertia of mercury.
Mercury was easy to obtain back then - I have a bottle of it - but I think it's frowned upon now. I really like Rockyracoon's idea of using a common reservoir and wish I'd done that. There's no way to get a bad reading with that setup and no calibration would be required. And if your gauge is tall enough, I'll bet any fluid, pretty much, would work.
1999 GL1500 SE
2002 Honda VT750 "ACE"