My current roof uses 1 x 6's not plywood. Therefore when I tie the new addition's roof into the existing roof there will be a visible line which will telegraph through the roof surface. To avoid this I need smooth out the transition. I'm thinking I need to lay my first piece of 7/16" plywood against the 3/4" and then over lay a single sheet of 1/8" underlayment boards and an extra layer of felt paper to ease the transition. Then at 4' out I'll delete the underlayment and use just a extra layer of felt. At the 8' mark I'll just be using the 7/16" with a single layer of felt and continue out for the rest of the new roof.
Put 1/8 shims on the rafter or cut tapers so it is more gentle. With 3 ply shingles it will not be bad.
Do not use flat bottom gables with scissor trusses.