You cant enchant boots or helmets with smithing. Only armor, gauntlets and jewelry. So you will only get a possible 20% at 100 enchanting.
The secret to getting items to the highest possible point is to use Enchanting Elixer's to get the Smithing Bonuses even higher. I can make something have 28% Smithing bonus. So there's Gauntlets, Armor, Necklace and Ring possible to get this Smithing enchantment (I believe so!).
On top of that, Blacksmith Elixer for an extra 50%.
You can make some scary strong items... Game breaking for sure.