Basically, a hotkey that, when pressed, will halve your current selection.
Units closest to the cursor should be prefered to minimize collisions.
Unit ratios should be preserved as much as possible. (100 bandits and 20 thugs would bud off into 50 and 10)
The key could be pressed multiple times to reduce the selection further, or, Alt and Ctrl keys could be used to work in 3/4th and 1/5th increments instead of 1/2.
In practice, this would allow for rapidly splitting forces into chunks, but further, would allow you to select a desired number of units when you have many clumped into a small area. (Let's say there's 20 wasps stacked somewhere and you want to select 5 of them, you'd just press the hotkey twice and give a command)
Also in cases of being zoomed way out, you could select your army, shift+click one type of unit, then use this hotkey to split off a force without moving the entire group.
I hope I explained it well, the goal is to allow for some finesse with unit selection quantities at all zoom levels.
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