What's a good way to terminate a metal valley at the top?
If the "w" type metal valley extends to the ridge there is a large open hole in the shape of a triangle at the top.
I've been cutting, or darting, the metal with tin snips to fold (mashing, really) the sheet metal over some caulking, nailing it down and running a tin or comp shingle over that and, then, running the ridge caps over the top.
Would it be better to extend the valley above the ridge or top of the house, whatever, then, cut to flatten out the metal for running over the ridge to the other side and nailing it down?
Not many people use sheet metal valleys these days and the art of sheet metal work is fading away. The apex of two meeting valley flashings is a two or three piece assembly that is soldered together depending on whether the valley is "V" or has a folded ridge ("W") profile. This not the same kind of soldering you would do with a regular soldering gun. Any sheet metal shop worthy of their name can fabricate one for you. All they need is the pitch of both the main roof and the gable as well as the width of exposure and the shingle overlay. Take a piece of your valley flashing so they can match the profile. Don't do it with caulk unless you want a guaranteed future leak.