Butcher paper test:

Upright firring strips with butcher paper stapled up between 'em would show you the splash pattern with a specific round at a specific distance.

For a rifle, I'd use frangible ammo and start outside of 50 yards and check as I worked my way closer.

For pistol work, I would use frangible ammo and work my way closer starting at 25 yards.

USPSA rules are 7 yards for steel (I think), IDPA is 10 yards. Doesn't matter what angle, dangle or mount.

I shoot it at 10 yards or more, still get a little jacket splash back. Good eyepro is a must.

Ideally, you want to hit it straight on and have all the splatter come off at a 20% angle from the flat face. That puts it hitting the ground 6-10 " in front of a 36" tall popper. Some will say angle it down, I like it straight on. Any deep pitting or craters will have it sending a lot of lead back at the shooter.

So with the 10 yard rule, tiny little 5" plates can be a PITA when practicing at speed. Get 8, 10, 12" plates. They are pricier but worth it .

Gringop