There is no need to stand behind the spinning prop when performing a carb balance. Here is the way I balance my carbs:
Not wanting to subject myself or any of my friends to standing near a rapidly turning propeller while looking at two carb balancing gauges and wanting to do both the engine operating monitoring and looking at the carb balancing gauges at the same time myself, I devised the following plan:
I positioned the carb balancing gauges inside of the cockpit by adding extensions to the two rubber hoses -- the extensions' were cut so that there is equal length between each gauge and the respective connection elbow on the intake manifold (not that having the lengths the same makes any difference at all). I ran the two extended rubber hoses through the open air vent on the copilot side and positioned the gauges on the copilot seat where they fall "readily to hand" as the British say.
Next, I tied a length of 50# monofilament fishing line to the top of each throttle arm and ran each line aft under the canopy fairing and into the cockpit where I terminated each line with a little "finger pull" loop.
Now I can start up the engine in the safety of the closed (and warm!) cockpit and gently pull on each loop to see what carb needs to be tweaked to balance the gauges. Then I shut down the engine, make the adjustment on the carb linkage, restart the engine and double-check my settings. Repeat, as necessary to achieve vacuum perfection.