Measuring for valley's

The old roof is 29 gauge exposed fasteners and the new section will have 26 gauge standing seam hidden fasteners .. I guess I will have to check with the manufacturer Thanks anyway Steve

I have a 60x20' cottage with an existing metal roof. I am now building a 20 x15 foot addition intersection off the middle of the cottage. The cottage has a 7/12 pitch roof and the addition has a 6/12 pitch I now need to cut the old metal back at the intersection to build my valley jack rafters and to then sheath that new section. How far up past the intersecting roof line to I cut back to allow for my valley's ? Also how wide of a valley should I use? Thanks in advance Steve