[sailingdog]

First, I would highly recommend you watch the following video regarding the use of a backstay adjuster.

To figure out when the backstay is at 20% you'll need a tension gauge. You'll also want a ruler or some other type of marked scale attached to the backstay adjuster in some manner, or marking the line that is used by it. Once you've used the tension gauge and marked the backstay adjuster for the 20% tension level, you'll have to experiment with the backstay to find what settings work the best for what combination of wind speed and point of sail.

 

[sailingdog]

OK. So if I understand the general consensus so far, I need to mark the backstay tensioning line at a location just before the system is slack (and maybe even tie a stopper knot in the line to ensure it cannot go totally slack), and also mark the line where the tension is "fully on" (at 20% breaking strength), and then experiment under various points of sail and wind conditions to see what works best (and when). The best method I have found so far to determine the 20% tension point is using the folding rule method described in the Selden mast book, attaching a 2 meter stick to the stay using tape and measuring the distance between the stay end and the stick. When the wire has stretched 1mm (the slack distance has increased ) that would equal 5% of breaking strength.
Sailing Dog, I like Alex's videos alot. They do give a good idea of what to do and why.

Yup...you got it in one.

Thanks for the responses, this site is great! If anyone disagrees with my summary or wants to add some additional advise, please do.
Thanks again,
Tom

Glad to help.