Uthoroc is correct (in fact everyone did it at close range, not just the British). I fear I have to take some of the blame for the fact that a "continuous broadside" rule exists as it was me that suggested to Andrea that something like that might be useful some time ago (2011?). I must admit I didn't pay much attention to how the rule was worked out as real life got in the way when later development including this rule took place, but some of the suggestions above sound OK. Actually you could almost make an argument for rounding up rather than down since the overall rate of fire would be slightly greater (rather than loading all guns and then firing each gun hammers away as fast as the gun crew can manage).

In fact I've found the email where I made my suggestion (it came up regarding the overall question of rate of fire and rate of damage):

Perhaps a compromise might be to include an option for the “continuous broadside”. This was a valid fire order during the period and was similar to independent fire of an infantry unit rather than shooting volleys. In practice, when engaging in this type of fire each gun would fire as soon as it was loaded and ready, so fire would be ragged but continuous. You could simulate this by allowing a ship with a continuous fire action to shoot every turn, but only using the reduced fire factor (if the target was in the central arc) or reduced factor -1 if in the bow or stern arc. And only allowing one side of the ship to fire in this manner.
That said, we've not used the rule in any of the games I've run so far.