A plague on theoreticians who've apparently never tied anyone in their entire lives offering advice. As a Dungeon Monitor I've often had to advise against this types of thing, once having to stop a scene completely because a couple of overenthusiastic and underexperienced guys had overlooked the fact that the (too ecstatic to notice) girl's toes had actually gone past blue to coal-black. Fortunately there was no lasting damage.
Dental floss CUTS. Fishing line CUTS. Cable ties not only cut off circulation, they also
cannot be undone. And toe rings, as any woman will tell you, have a gap in the metal so they can be adjusted to fit, and thus will pull apart under strain.
Shoelaces or something of a similar width, 1/8" up to around 1/4" will work, but must be wrapped several times to diffuse the pressure then cinched and tied to the ankle rope or stocks fitting. Here's an example.
Part 1, toes:
Part 2, ankles:
Part 3, toes attached to ankle rope. If there is tension on this connecting rope it will be quite difficult to wiggle out of the toe ropes.
And always remember that it's the turns of rope holding the limb or whatever in place, NEVER the knots. Those are only to secure the ropes themselves. That way you'll never cut off circulation.
Feet courtesy of KittenToes; all ropes in this example are 1/4".