So even if my skill of enchantement was 100, if I disenchanted a armor with 20% extra dmg with bows, it would give me the same base enchantement (13%) (if I have no perks)?
No. Once you level up your enchantment skill you will actually produce much higher results than 13%. My enchantment is now level 80 and allowed me to apply the perk to allow 100% improved enchantment. I can produce much better results. So a bow item that only originally allowed 13% will now easiy give me over 40% better bow damage. As you level up your enchantment skills and add perks to "enchantments" the results are much higher. Plus make sure you use a GRAND soul gem filled with GRAND soul to get the best results. Also keep in mind you can apply the enchantment knowledge to multiple items. I have a BOW ring and a BOW necklace that I put on whenever I am using my bow. It will easily bring a bow from about 35 damage to over 80 damage. Even higher if you enchant more bow items.
I'm really glad you explained that as I didn't know that the items I enchanted would stack. Thanks for that.
NP. Yes you can stack multiple items. In fact I have a RING and NECKLACE for just about every occasion. Bow strength, magica, smithing, money discount, fire resistance, weight reduction, frost reduction, increase shield strength, and much more. At the click of a button I just change my ring and necklace for the event. :) You can also add the enchantments to light clothes too. I have boots, gloves, a necklace, and a ring that I put on that all add over 120 extra carrying weight to myself and I have an entire set for my follower. Works great for carrying loot out of caves once all the enemies are dead. Enchanting is probably one of the most important skills to learn in Skyrim in my opinion.