Seems to me, 15 feet was where I set my screens, on a tri-pod. I do not recall having my screens blow over from the muzzle blast.
That did happen with the wind though, when using a bulky Chrony instead of just the sky-screens on the Oehler & PACT timers I used for years.
I shot loads to 330gr. in my .69 (loading error) and 200gr. in my .58, but never blew over the screens. I think perhaps it could happen, if the
ball went just over the screen openings, so the blast was low to the machine (Chrony).
MartyB - no trouble with residue on the "eyes" - ever.