re: buttercream HELP!!

I live in upstate New York. It gets very humid here during the summer months. I currently use the 50/50 butter to shortening ratio with milk and powdered sugar. Is there a better option? (ie ratio butter to shortening.) I was thinking like 40/60? How about adding meringue powder? To ensure once I smooth my frosting on a wedding cake it will stay hard?

Party Cakes