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Daox,
My goal was to have R-35 of cellulose at the transition between the cellulose and the polyurethane foam, so that is 10 in thick. How far back from the wall header depends on your roof pitch. Mine is pretty shalow, a 3:1 pitch. The final block design was shaped like a wedge with the nose cut off with 2 small 1/2 inch strips along the sloping face to create an air chanel. I squirted a heathy dose of Great Stuff Foam on the top of the exterior wall header when I installed it. It works great. NO icicles anywherte, and a thermal scan showed no cold spots. It was a pain to install, but I kinda overdo things when I never want to go back to a spot in the house. An easier solution is to contact a polyurethane spray foam installer and have they do your attic in the trouble spots, like wall headers, recessed lights, and the eves. It will be cheaper for a pro to install the spray foam than the DIY approach because of the HUGE markup for the DIY kits, like Tigerfoam. I went with a pro installer when I did my exterior remodel, because his quote came in under the DIY material cost
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