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The rules I use are more concerned with the number of 'bases' rather than figures. Black Powder, General D Armee and Honours of War current favorite sets
Essentially for a standard battalion four bases works, and for WSS and Napoleonics 6 figs in two rows per base looks right to us (40mm aware bases)
For Napoleonics then would throw in additional bases of Skirmishers but more at a Brigade level. 2 or 3 figures to a base.
For units that were larger or small add base or two
We like big games so that works, if doing a smaller campaign such as War of 1812 may be tempted to use larger battalion (say36) for the aesthetics.
I use Carnage and Glory computer moderated rules. My battalions
are 4-6 figures, my cavalry regiments are 4-6 figures, my artillery
is 1 gun per battery. Carnage and Glory has a system that keeps
track of exactly how many men you have fit, are wounded, or missing
so basing is more for color than counting.