You do realize that a percolator is the absolute worst way to brew coffee right?
Youd be much better off making up several pots in a drip brewer and putting them in an air pot.
Another alternative would be to make up a batch of CP and have the percolator there just for its hot water to make a cuppa with. :)
If you do insist on using a percolator use a pretty coarse grind. Coarser than for a drip brew. Probably somewhere around the grind used for a french press pot.
For a mix ratio for strong coffee somewhere around a 6:1 water:coffee should work.
Java "Kill the percolator!" phile