Yeah, I know there’s a custom split, but then that adds another step - you have to then type in a number.
A pretty good interim solution would be to have canned splits for 2-10 units (even though that would increase the size of the menu), with the default action being to move just one unit off the stack.
I think that would be more intuitive: you can add units by dragging them on to a stack, so you should be able to split units off by dragging them off.
Another way to go would be to just have the right-click immediately open a “custom split” dialog, so it’s just “right-click, type #, enter” but I know you also have other options in the right-click menu.
I agree though, even in Battlemap, moving 3 or 4 units is also a bit of a pain.
The most flexible canned splits would be a binary progression: 1, 2, 4, 8, 16 etc.
Another suggestion would be to have three mouse modes that you select via right-click: Add, Delete or Split, and then you have some other way of influencing how much each click should add, delete or split off.