There isn't really a "best" ratio, what I do is I look at the battles in progress, and I look at the armor/piercing specs of enemy infantry and armor divisions. And I just adjust my breakthrough and exploitation armor/motor divisions to have enough armor so their piercing can' penetrate, and other then that I just go for as much org/breakthrough/soft attack as I can squeeze into a 10 or 20 wide division.
*edit* I even keep usually about 5-10 reserve 10 wide divisions with just light TDs motorized and support anti tank guns to use as a mobile reserve to stop enemy armor breakthroughs