Heres a guide on how to make your anvil percentage:
1. First you go to the anvil and put ur item and a BUS Scroll (on a High Class Item) on the anvil and click "OK". A Pop-up will say, "Item may break while upgrade, are you sure?". Then, you wait until the colors around the anvil are
red and only RED or else this will not work, then you click OK as fast as you can and the item should have a higher percentage of successing. I know this from experience because i have a +6/10 full Chitin Rogue set and +6/60 Mirage Daggers from it.
2. After the server resets, go on a server with no people in it (for example, Xigenon 3), and fail 10 items, then you can upgrade an item to +8, like a shell item, or a weapon.
3. Break Priest Chitin (for High Class Items, unless you're a priest) and then upgrade the item you want.
I want my premium please!
EDIT: Oh btw, if anyone knows ELT WaitForMe, thats how he got the full +7/12 and +8/15 shell and +8/80 Shards by using method 2, and yeah there is a difference in success for all of them.