Hi there. I’m new to the site, but happy to see a place to talk about cakes! I am making a 3-tier cake this summer; 16", 12" and 8". According to a site I found, I would need 15lbs. I was reading the 5 lb box of Wilton and it says it will cover a 10", 8" and 6". In my head that would mean 2 boxes would work for me which is 10 lbs. Any suggestions?