This is a bit of a skill and experience thing.

It also depends on the shoes. For example, KoolStop have a built in Toe-in plow point in the back and will tend to squeal until this breaks in. Adding enough toe-in on top of this "plow" results in excess toe-in and spongy brakes.

On of the easiest ways to deal with squeal without resorting to excess toe-in is to remove the wheel and use a coarse file to remove (chamfer) the back corner of the shoe so it skis above the rim rather than dig in. (imagine the difference between pushing a sled or a box through the snow). This and natural break in will manage squeal very effectively, and odds are the brake will only squeal occasionally when the weather changes. (mine sometimes squeal on damp mornings, and stop within an hour or so).