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You have the basic idea. The backstay tension will affect the genoa and main. One thing is does is allow the main to "twist off" at the top opening up the leech of the main. The more tension on the backstay the more the main will be twist off and be de-powered. This can be helpful in a blow. The other thing is does is affect headstay sag and therefore genoa shape. So again if you are trying to go upwind (maximizing your pointing ability) in a blow, that is when you would see maximum backstay tension, and therefore headstay tension. On a racing boat in gusty wind the backstay tension and the traveler position may be under constant adjustment. Every time a gust comes along the backstay tension will be increased to "spill" some air and the traveler may be dropped as well. After the gust the backstay would be eased to power up again. The correct settings for your boat can only be determined by experience. In a medium steady breeze go from min to max tension and back a number of times while close hauled. Carefully watch the shape of the main including the leech shape and draft position, and pointing ability.

As far as near zero to 20% goes, on a racing boat you will often see a stick or some sort (perhaps piece of batten material a few feet long) attached to something stationary at the backstay adjustment point. You will have a line drawn on the batten and on a movable part of the backstay that line up when the tension is near zero. The backstay will be very close to completely slack and "floppy" at this point. I've never seen a rig fall down by releasing all tension when the boat is at the dock. However, if you're too worried to do that ask a rigger for help. Then draw another line at max. tension. You can pull hard on the adjustment but don't run the line to a winch or something like that. It was designed with a 6 to 1 and I'm sure the assumption was that a typical human would be pulling on the line. Again, your going to have to ask a rigger if you want a more exact calculation, or perhaps install a tension gauge but I don't think this is needed. Anyway, with the stick and a min and max line, now you have a reference while sailing.
 

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While sailing upwind stand where you can sight up the mast and watch how it changes shape when going from min to max settings. Do the same at the headstay. You'll get the idea.

Backstay tension does affect headstay tension on a fractional rig boat, but not to the degree it does on a masthead boat. The amount of effect depends on the spreader configuration and other elements of your rig. See it for yourself.
 
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