Gerald,
Buy "1864 Field Artillery Tactics" published by the War Department. There is an extensive section on aiming and "line of sight", "point blank" and the effects of windage and terrain. We call it "The Red Book". I think your technique will work well enough. As the most common field pieces were on Type I and II carriages. The History Channel did a show several years ago that showed a team from PA (I believe) working a ten pounder Parrott. They put sixteen rounds in a row in 4x8 sheet of plywood at 1000 yards. The Gunner said that the sight blade completely blocked out the target at that range. But he also said he was aiming way right and that the round travelled almost 40 feet above the muzzle in flight. Artillerists were highly trained in the arts of math and geometry. There is a demonstration that they do at some shoots that replicates a drill done back in the day. Tear off the front page of the newspaper and hang it at 1000 yards. This same PA team put three rounds within a 16 inch circle on that paper. I read somewhere (can not cite it) that one of Pelhams guns at Fredericksburg aimed at and broke the flag staff of the unit in their front. I believe all of Pelham's Battery consisted of smoothbore Napoleons. Pretty impressive.I also have a diary by a Sgt. White of the Third Richmond Howitzers (detachment number 4, a Napoleon) that states they got off eleven rounds in the first minute of a frontal assault by Federal infantry at Pole Green Church in Hanover, VA. That tube had to be glowing red. I'd love to read the results and conclusions of your research.

Keith