"No aids" I take to mean, no milling reticle, just a fine cross hair- and no fore-knowledge of the target size. Blithely aiming center mass, a 147 eld-m from a 6.5 creedmoor will put a bullet on that sized target with that wind up to about 275 yards. Knowing the target size, but no way to range (or estimate range), you could aim 'top of plate' and make a hit out to about 375. At these distances, "favoring into the wind" is sufficient to negate the effects of wind. Given a reasonable shooting position, I'd say these would be 10 for 10 propositions. Beyond that would require some method to estimate a range.
We know the target size, 16"W x 24" tall, it's bullet drop like you say. I have access to a few 1K+ ranges, shoot on one long enough, does not seem far. But if you stood up and tried to find 600 yards directly behind you, whole different game. So location plays hard, judging 100 yards while on the ground on a football field, seems like twice as far as a target at 100 at a familiar range, gets worse as you go out w/o a range finder.
Your example was 6.5 creed, but with most 6mm's shot today in comps, a top plate hold would hit to 450 w/o much issue, with a 100 yard zero. I don't want to place a bet, but with any depth perception, tricky in all locations, it would be tough to overshoot at 500, so the play in the solution will be low.
I suck at reticle ranging, one day we tried ranging big dog steel headbanger targets starting at 550 on the odd numbers to 1050, 650,750, 850, etc..., at 750 I was out of the game, known distance, just could not open an app to verify, and by my misses, a full size IPSC would not have upped my success ratio.
So all day, everyday, 450 is my answer, given you don't overshoot.
