Nail position is not as critical with ridge cap as it is in the plane of the roof because of the bend, which keeps the ridge from being able to move in high-wind situations that would lift flat shingles. At the tar line is probably fine. I wouldn't go much either way because you reduce holding power or increase water leak potential.
When in doubt, follow the manufacturer's recommendation for the normal nail position, and lap the shingles at least an inch past the nail. When possible, lay them out so that the lap falls downstream with respect to prevailing winds.
Be aware that 1-1/2" nails are typically not long enough for ridge cap due to layering and possible ridge vent gap.