I discovered that my state says the roof must be insulated to R30 (I thought it was R40). This info changes my plans- I no longer need to shave these panels flat because I no longer have to get 9" of foam in place- I only need 7 or 8 inches- this gives me more vertical space between the foam and the plywood. It also means the thickness/height of the foam can vary more, so I don't need to worry about the foam being perfectly flat.
But I also need to fit a 48" wide panel in a 46.5" space (sleepers/ribs must be 48" on center to allow the plywood to attach). So I turned my foam cutter sideways.
Answers the problem not being able to measure how far to cut the roof decking while up on the roof. This would be easier with a man-lift, but with just one person and no man-lift, it becomes dangerous to try to lean out over the edge and measure how far the rafter is from the edge. Roof decking is now trimmed.