IPSA pistol rules say 7 meters... its a DQ for "unsafe gun handling" if you shoot closer. With this said, I have seen lots of incidents of low-velocity splatter hitting people, sometimes causing minor injuries (superficial cuts). Usually the risk of serious injury is small provided people are wearing eye protection (including spectators).
The actual amount of backsplash/ricochet, and thus the minimum safe distance, depends on several factors:
- The characteristics of the bullet being fired (shape, weight, construction).
- The characteristics of the target (surface topography, hardness, resistance to movement).
- The specifics of the impact (angle of incidence, impact velocity).
In general, you want to avoid shooting steel with very hard bullets; armor piercing/SS106, steel jacketed etc. = bad, soft lead or HP = good. The steel target wants to be hard and flat (no curvature or surface craters), mounted on a stand with some "give", perpendicular to the bullet trajectory in the horizontal axis and angled slightly downwards in the vertical axis so that the splatter is deflected into the ground.
Common sense and 100% use of safety glasses should see you through.