Ah.
Hi again,
The Hunter in the video is a B&R with a backstay. For your boat, how much tension goes onto the backstay will be determined by how much the spreaders are swept, how stiff the mast is, and whether or not you have runners; 20% or so could work on a tall fraction, where the backstay might be counted on for most of the forestay tune, though even this might be moderated by the relative sizes of the two wires. Remember, what we are after here is not percentages of backstay loads, but straightness of the forestay. If you get a good shape with intermediates and/or runners and/or a jumper, plus 10% on the backstay, then you're home.
Also bear in mind that, on a fractional rig, the backstay's other important function is to shape the mast. On many rigs, tensioning the backstay does little to tension the forestay; instead the mast will bend, with the forestay as a fulcrum. Adjustable backstays are just about mandatory on fractional rigs.
Selden's free tuning brochure has nice details on this. You can get one from my shop for S&H.
Let us know how the sail goes!
Fair leads,
Brion Toss
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