In truth,the calculation methods for loading a steel rope,are very complicated. For an application,such as yours,experience would allow easy selection,with a factor of safety included.
The SWL on a cable,would,in lifting,hoisting and winching,be at least 5 times less than its' possible breaking strain. Some applications requiring a higher factor of safety.
A SWL in extension,with formed anchors,would not be converted easily,to the load on a wire under tension,with the load perpendicular,along its' length.
You can test this,easily,with a piece of polyprop (blue) duct drawstring,where even a fat dude,can suspend confidently off a single piece,but tie it between two trees,and bounce in the middle...it will break.
This is due to the "advantage" principle,a load at ninety degrees,at the mid-point,applies.
Loads of this nature,on a suspension wire,would include snow,ice,tree matter,roosting birds etc,and give cause to ensure fixings up to the job.
I remember seeing the effect of this,where a local council had fitted some real nice SS wire and gripple fasteners,between two lamp posts,and loaded it up with Christmas illuminations,to the point of bending one steel post,permanently.
