Haythornthwaite's books Napoleonic Infantry Tactics and the Napoleonic Wars Sourcebook both discuss this referring to British firing tests during the Napoleonic wars.

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They estimated that at 50 meters, you had a 50% chance of hitting a man-sized target.

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It should be noted that military-issue muskets were often of poor quality and outside of the British and French armies, live firing training was limited (or even non existent).

As you say most training was in volley fire (the reason why the order is given as "Present" and not "Aim"), men skirmishing were expected to fire at individual targets and in most armies all units were expected to be able to skirmish to at least some extent.

The book also notes that there's various short-cuts you can take in loading the weapon, which improves speed but may affect accuracy.

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My conclusion based on my reading would be that slightly better accuracy than the tests is possible, provided you have a good quality weapon that is loaded carefully and you have trained in accurate firing, but it'll still be fairly dire by modern standards.