FWIW...
Soft shackles are 4 times the material (IE the single rope is doubled and doubled again) and by far the weakest point is the knot which I allocate as 50% of B.S. so, based on my experience with them on vessels over many years I rate my homemade Softies as follows;
For winching, rigging, redirects, towing etc IE non-kinetic applications 2 x B.S. ...That means 20 tons for 10mm, 15 tons for 8mm and 8 tons for 6mm (I don't use 30 ton 12mm for winching as there is no point)
For snatching IE kinetic applications 1 x B.S. ... that means 15 tons for 12mm, 10 tons for 10mm and 6 tons for 8mm (I don't use 6mm for kinetics)
Given most 4by Winches are 4 to 5 tons max line pull IE 8,000Lb to 12,000Lb and snatchies usually 8,000 Lb (less than or around 4 ton for 4X ) the winch stalls out or the snatchie lets go.
I tend to use 8mm (occasionaly 10mm) for snatching and 6 or 8mm for winching/towing/dragging.
Sooo... I carry 2x10mm, 2x8mm and 4x6mm (2 short and 2 long) in my "touring recovery kit"
I have a couple of lengths of random sheath (3 or 4 inches long) that I slide over the softie for use on recovery points with sharp edges if I can be bothered.
Bottom line is in most applications the softie is operating at under 1/4 to maybe 10% B.S. of the line it is made from...or 4 times to 10 times safety factor if you like... plus Dyneema is bloody tough so any metal edge has to be pretty damn sharp to cut it and even a few coats of paint or a slightest chamfer and the softie doesn't even notice it.
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