We were out today with the flintlocks where we always hunt deer in rifle season and met for lunch at our usual spot where we can see across an old strip cut about 250yds. We were kidding about taking a shot across there if one came out of the pines and one guy told me I couldn't hit a sheet of plywood that far away. Well now I have $20.00 on a bet saying I can. I'm shooting a .54 patched round ball Lyman GPR with 90 grains of 2F. How high would you hold on a 4'x8' sheet of plywood at 225yds. ? Right now i'm a couple inches low at 100yds.
Those who hammer their guns into plows, will plow for those who didn't
OK, so using QuickLoad to predict your velocity, it estimates you're getting about 1585 at the muzzle. Then feeding that to the roundball trajectory calculator, I fiddled with the zero distance till I got a 2" drop at 100 (assuming when you said "a couple inches low at 100" that 2" low at 100 was in the ballpark.).
End result = estimated drop of 73.4" from point of aim. So, aim 6 feet above what you want to hit.
If this were me, I'd put it on end (long edge vertical) and hold at the top edge or slightly above it. A 4x8 sheet set vertical gives you plenty of drop just aiming at the top edge, but if you shade it about 2' above the top edge (1/4 the height of the plywood for aiming reference when you're looking at it), that would land your ball pretty near to center of the sheet vertically. I would think within a foot either way, anyway.
I would hold on the top of the 8' vertical or maybe a slight hold over and a drop of 6' at 250 should still give you a 2' margin on the vertical drop but don't forget to factor in a 5' wind drift at 10 miles per hour so calculate that in. Your going to have to hold off the sheet into the wind or you will miss. I think you can hit it vertically easy but the wind is what you need to watch.
Go to the Hornady website and use their ballistic calculator; it's free. A .530 ball is 224 grains with a ballistic coefficient of .075. Use the advanced tag (click on basic and select advanced) for entering the expected wind drift. I love bets like these. Do your study and drop the hammer. TDD's velocity is about right for your barrel length and charge(1585)
As said above I would worry more about the left or right than the drop. The distance stated for a 10 mph wind is assuming the gun is perfectly level when you shoot. Is this going to be off sticks or a rest? Shouldn't be a problem, off hand is something else! I also would want an exact range and not just "about" 250 yds.