It depends on if cash is an issue. If you can afford it, something like 60/40 or infantry/cavalry is quite effective and generally prevents insufficient support, provided that your unit type is 50/50 max cavalry.
However, if money is an issue, the old standby of Combat Width minus 2/4/6 inf + 2/4/6 cav is still fine.
EDIT: Also, if you have NIs giving at least 20% cavalry combat ability, you should probably always use as much as you can get away with because that's a pretty huge bonus to cavalry performance. Especially if you're Poland and its +33% mounted gods.
On the flip side, if you have huge +infantry combat ability and not cavalry combat ability like Sweden and Prussia, you should probably use relatively minimal amounts of cavalry and lots of infantry.