1:60 rule

[mikehallam]

I radian is 57.3 degrees i.e. nearly 60.
[A radian is an arc of radius R to the length R round a circle's circumference.
Near enough to a chord.

Or think of an equilateral triangle, i.e. equal sides and equal (60) degrees each inside corner.

It's simplified calc. as one of many ad hoc tools for sensibly assessing & correcting a course when map reading.

Mike Hallam.