I put together the round ball calculator in question.
I used the ballistics data from the British round-nose cannon reference round (19th and early 20th Century data) as a starting point and adjusted the results to account for the lower sectional density of the round ball. The output was 'confirmed' using a chronograph measuring velocities up to 50 yards, and they seemed pretty close. I would have used longer distances, but I didn't want to shoot up my chronograph.
The equations used are basic physics, but with the British empirical data as a basis. As for the 'paradox' of a higher velocity leading to more wind drift, this is a result of the round ball characteristics. Wind blowing across the ball's path and the ball's forward velocity combine as a velocity vector which results in both a rearward and sideways 'push' whose force, at a minimum, is proportional to the square of the velocity (higher at supersonic speeds). Thus, the wind force will be greatly magnified the higher the velocity. This means the maximum sideways acceleration of the ball occurs near the muzzle. The forward velocity of a round ball drops off very rapidly, but the sideways velocity does not; this is why time of flight has such a profound effect in the wind. If the velocity of the projectile was better maintained, this effect would not happen.
Hope this explanation helps; I make no claims for the calculator other than it might be a useful starting point. As others have mentioned, there is no substitute for real-world tests using YOUR rifle.