Better yet, a purpose-built interface window for managing drone balance between all controllers in an easy and intuitive manner, like a bar graph where each line is a controller, and the bar is a bunch of drone icons. If you click and drag the last drone in the bar, you just move the one. Half the bar, you move half. The last three, you move three.
Or... for less UI programming an even simpler way would be to set an ideal number for each controller using simple up and down carats, then the controllers use an algorithm to determine who gets how many. The algorithm would be the most complex part of this solution, as it should be something other than just linear... you would want higher number controllers (relative to their possible max so you don't totally hose RC controllers, or just keep them separate) to get closer to their set point, but also not totally devastate low priority ones.