Your bonus lucky hit chance is applied to your respective skill’s base lucky hit chance.
As an example, say you’re using a core skill that has a 20% lucky hit chance by default. This means that your skill will lucky hit 20% of the time. If you have 100% lucky hit on gear, that would make the core skill lucky hit 40% of the time (roughly).
Now your amulet says that 15% of those lucky hits will heal you. So overall you would have a 6% chance to heal with every attack (15% of 40% = 6).