: Anybody got ratios for sliders--rules of thumb, anything like that?
: The lines get looser as you move the sliders together, clearly,
: but how do you figure things out so that they're really tight
: when the sliders are spread apart?

The greater the difference between the hole spacing at the gunwales vs. in the slider(s), the more tightening effect you get as you push the sliders apart. A difference of 3x seems to work pretty well and more would just increase the effect, but there's probably a point at which you start having problems with the sliders not staying tight.

: What keeps the sliders from creeping back together under load?

Friction, plain and simple. I always drill the holes undersize, so I have to twist the cord to get it through. If you use wooden sliders, they'll expand slightly when they get wet, which increases the friction somewhat. Use hardwood for the sliders to prevent compression or excessive wear in the holes and they'll last for years. I typically use scraps of mahogany decking lumber. I haven't tried bone or plastic sliders, so I can't comment on how they work.