Thanks so very much for both your replies. By trial and much error today we arrved at the 30 degree backing angle. It seems confusing to set the saw on 30 degree in order to get a 35.2 degree bevel plane with the top of the jack rafter, which is coming down and approaching the valley on a 45 degree angle. From one of those websites I also confirmed my calculated 83.125 inch spacing between jacks along the valley - 4 foot on centers. I am feeling much more confident that this will work. I'll try to post some photos when erected. Thanks again.