That is right. Most tanks are designed, that the frontal armor is the thickest and many of the WW2 tanks where very weak at the top. They where basically design

That is right. Most tanks are designed, that the frontal armor is the thickest and many of the WW2 tanks where very weak at the top. They where basically designed for frontal tank vs. tank (or anti-tank) attacks. Today's tanks have to take air attacks into account -- with much better top armor but also better armor at the other sides too. Your calculation is right, I guess, but still 120mm has just overwhelmingly more hitting power than 30mm rounds.