You want to be about two inches above the very center of the valley itself.
How to tie existing roof shingles to shingles.
After framing and sheathing the new addition s roof snap a line over the existing roof shingles.
I e how big the last ones are i would recommend that you remove enough of the old shingles so that you don t leave the plywood seam the tar paper seam and the shingle seam all stacked on top of one another.
Other than that you would just follow the pattern that is already there.
I will be creating a new valley on one side and tieing into the existing shingles on the other.
It really just depends on how the existing roof shingles ended.
I am about to put an addition on my house and the only thing that is keeping me from doing the shingles myself is the tie in to the existing.
To tie into the side of an existing roof put a new roof truss on the walls of the addition brace it upright then snap chalklines on the shingles to mark the center line of the new roof valleys where old and new roofs will connect.
Up from the valley center and cut through the shingles and underlayment.