rain diverter install
I hope your not thinking about using a piece of vinyl J moulding for this?
Where the deverter gets nailed to the roof it needs to be at least 3" up under the edge of the shingle.
A simple L shaped piece of coil stock is the simplest way to do it.
I do a 1/2 seam, then make the next bend at 1" and cut it off at 3 or 4".
The seam adds strength to the raised area.