How close to roof should downspout be?

Any unusual wear on a roof will eventually “show” differently. Depending on the size of the upper roof, you could have a substantial amount of water splashing onto the lower roof.

Water will create a “wash” on the granules. That is to say, water running over the shingles will wash more granules off than where just the rain hits the roof.

We use a 10”x18” “splash pan” made of stainless steel with its edges turned up 2” on 3 sides. It is then installed with mastic under the downspout opening. It will protect the shingles directly under the downspout opening and allow the rain water to spread out before running down the lower shingles.

Just extend the downspout with another section and put another elbow on it so it dumps directly into the lower gutter.

That would eliminate the water wearing the roof prematurely or the need to alter the roof.