Hopefully someone with a Dufour
will get you the answer so you can get them "just right". Barring that, it's usually not too big a deal. Take a look at some 35 pictures... are the upper wires perpendicular to the water
? or a little less? You could get pretty close knowing that, measuring from the mast
step to the chainplate athwartships and calculating the length. The reason I say this is I designed my own rig on one boat i built. Then I took the design and had a noted marine
Architect analyze it and offer advice. I learned a lot. Basically it makes very little difference if the spreaders are up or down a foot or two and if they are +/- a couple inches in length.