When should I use Roadhog's alternate fire?

Roadhog's Scrap Gun has two modes of firing. A short-range shotgun-like primary fire, and a long-range shrapnel ball secondary fire.

Scrap Gun primary fire stats

Scrap Gun secondary fire stats

At what range does Roadhog's secondary fire start dealing more damage to a single target than his primary fire?

Well the "alternate fire" is just like his "primary fire" except with a longer range. It launches a ball of scrap, and after a short distance, it explodes into a normal shotgun blast. It's meant to be used to kill or harrass people who are outside of your regular primary fire range, assuming your hook isn't available at the time.

A similar question has already been answered on Reddit, where someone has posted a diagram portraying the use of Roadhog's "alternate fire", credit to Redditor _SpliceD;