In Smoothbore Musketry, Larry Irons shares all sorts of tidbits on the topic, with an eye to wargaming. He presents some coldblooded calculations:
At the Battle of Blenheim (1704) the British with five battalions attacked the French fortified positions along a front of 750 yds. The French had approximately 4,000 fusiliers deployed along 900 yds. The French opened fire at 30 yards with a single devastating volley causing 33 percent casualties to the British attacking force. This came to approximately 800 casualties. Therefore 20 percent of the French rounds were effective. If we assume that 15 percent of the French muskets misfired, this gives an effective rate of 23 to 24 percent of those muskets that actually fired.At the Battle of Fontenoy (1745) five British battalions with a total strength of 2,500 men, less a few hundred men due to French artillery fire, let loose a volley at 30 yards against an attacking force of five French battalions. The British volley caused 600 casualties to the French. This would mean that the British muskets were hitting with an effective rate of 25 percent.
At the Battle of Minden (1759) Hughes estimates that the effectiveness of musketry by both British and French was less than two percent per volley. In this battle the French and British engaged at much longer ranges, 100 to 150 yards. At the Battle of Albuera (1811) a French divisional column attacked the British position. The British muskets averaged a two-percent effectiveness rating at that battle at a range of 100 to 150 yards. However, at the same battle on the French left flank, the average effectiveness was about 5-1/2 percent per volley for both sides. Hughes concluded that at Albuera the actual effectiveness dropped off rapidly with range between 30 and 200 yards. He also stated that smoke on the battlefield often obscured the aim of the shooters, which would lower the effectiveness dramatically.