So what basing do you need for our upcoming black powder "mass skirmish" rules?
Stands with 3-5 figures on each essentially: Likely a basing standard you already have.
Cavalry can be mounted with 2-3 figures per stand.
In both cases, all representation is 1-to-1.
Some players may wish to base good troops with only 3 figures per stand while fresh recruits get 5.
Since all combat is by unit, not by figure, this is a good way of portraying battle hardened veterans.
If you have individually based figures, you can put them in clusters of 3 and that should work fairly well, though it can be a bit fiddly.
The goal is that you can take your existing armies from quite a few games and use them "as is" without having to worry about rebasing anything.