I believe the 'standard' is generally 25 yards for the .410 using a 30" circle centered over the heaviest concentration of pellet strikes.

I have used 30 yards on the plate as being the maximm effective test distance, as it gives me a better visual undertanding of the cartridges limitation. A few shots at the plate from 35 yards w/your better ammunition will generally give you all the visuals needed to understand the serious 'luck' factor involved in cleanly bagging game at that distance, it is already evident at 30 yards with many loads.

The link below, though not specifically addressing the .410 says it best:

http://www.billhanusbirdguns.com/archive29.html

FWIW, I found the Fiocchi 11/16oz. 3" w#7.5 shot and the shorter hulled Winchester Sporting Clays AA's w/#8.5 shot to provide some of the best patterns I've seen from factory .410's for field use, i.e., testing at 30 yards.

The newer Remington STS skeet loads also threw excellent 21 yard patterns from a couple of skeet & one tubed gun I shot a few rounds through, but it was not extensive testing on my part. I will quickly admit that I mostly shoot inferior reloads at skeet w/the .410 as they are not the limiting factor for my level of execution;-)