There's no need for trimming shingles to force a fit. Drill one pilot hole into the rafter, backfill it with appropriate sealant, slide the flashing beneath the shingle in the course above, drive the bolt home, and it's done.
The 12" x 12" Classic Composition Mount flashing is designed to fit within a standard 5 to 5.5-inch course and the exposed course should be no greater than 5.75 inches. If the exposure is greater than this, use theClassic Shake Mountas an alternative. There's no shingle cutting and no shingle distortion caused by trying to force a fit – just a permanent, watertight mount every time.