As far as I know, the enchantment rate decreases also with enchantment rate level before +10, so for maximum chance for an eternal weapon that is +1 you would need a L106 enchantment stone and so on.
"The sum of the level and enchantment level will give the player a number. If we take as an example an item of level 60 which has been enchanted to +7, the sum will be 67. Then, the player takes in account the grade. Eternal equipment will require 40 levels to reach the cap, while Mythic items require 50. As such, taking that the example item as an eternal, consider adding up 67 to 40, making it 107; therefore making the Purple enchantment stone <L107 Enchantment Stone> the most optimal."