How important is level?

why bother trying to get exact level when flagging a swale? Once the earth is dug, it won't be exactly level.

 

true, but water does seek it's own level so the closer you get it to
level the less water movement along the swale happens. which
keeps erosion from happening...

 

Ideally would would traverse the dug swale again to level out any high/low spots. During lighter rainfall events this will spread the retained water further rather than pooling.

 

I think it depends on your goals for the swales, how much water you annually receive and what sort of volume a catastrophic storm might produce.
I use .5 degrees of down slope on our swales and they turn at back and forth to handle the up to 7 inch in 3 hour rains we sometimes receive.

This is because I don't live in Australia, where yeoman's lived and developed his system (which works beautifully there, but not so much here in the USA).