At the risk of confusing the issue, a bit more background.
For angles measured in radians, rather than degrees, the approximate relationship
sin (angle) = angle
(which holds for small angles) can be used to simplify the off track triangle geometry.
The basic geometry is
tan (TE) = distance off track / distance along track
which, for reasonably small angles, can be approximated by sin (TE). This, in turn, can be approximated by the angle in radians.
Now, it works out that, for small angles, this corresponds to a distance off track of x nm for a track distance of 57.3 nm (which, naturally enough, is then rounded off to 60 nm ... hence 1:60) if the included angle is x degrees.
For instance
1 deg = 0.017453293 radian, and
sin(1 deg) = 0.017452406
so the error is sensibly not too much of a problem.
Limitation is that the errors get out of hand as the angle gets bigger so, for practical use, 1:60 is useful for angles up to around 15 degrees or so .. ie normal DR nav problems.