I'd recommend using a minimum of one roll 36" wide of ice and water shield the length of the transition (about 76'), overlapping at roll ends about 6" min.. It comes in 33' rolls or 66' rolls, you'll need to buy 3-33' rolls. You can apply some of the extra to the rake edges where the two roofs intersect to reduce the possibility of water shooting out sideways, I'd probably cut it and put down 12-18" or so on each side to the peak on the existing roof, over the ice and water already applied at the transition. This will mean you'll have a partial roll leftover, but those are the breaks. Any metalwork would be done after application of the ice and water. WHY? The ice and water shield wraps around nails as they penetrate the roof surface, providing a leakproof seal.
I am honestly a little confused about why you are using any metal at all, perhaps I'm not getting something clear in my head about the job.
The best I&W shield is made by Grace, and it will typically wrap around a 1" nail without tearing at the nailpoint (hard to believe until you see it).