You want to be about two inches above the very center of the valley itself.
How to tie existing roof shingles to shingles.
After the addition roof is framed and sheathed snap a line on the existing roof shingles 2 in.
After framing and sheathing the new addition s roof snap a line over the existing roof shingles.
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.
The easiest way to tie in the new shingles with the old is with a cut valley see four ways to shingle a valley.
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.
It really just depends on how the existing roof shingles ended.
Up from the valley center and cut through the shingles and underlayment.
I will be creating a new valley on one side and tieing into the existing shingles on the other.