What is best way of joining with formations?

An advice would be greatly appreciated.

I'm struggling with formations management UI during bear join for example.

I have a preset formation, but the number of troops may vary due to available space, if I press "reset" at the bottom left 2 times it will set troops to 0, and then to fill them to available rally size. But 2 seconds of this is often too long, and the rally is full.

How do you guys manage formations and quick joins? What is the best way of handling it?