A clear-cut rule, so that it is easy to make a decision.
bright line
A clear distinction in the context of a legal, moral, ethical, or anal…
cross the line
To overstep a boundary, rule, or limit; to go too far or do something …
draw the line
To set a boundary, rule, or limit, especially on what one will tolerat…
see one's way clear to
To manage to commit oneself to (a decision or course of action).