So, I did some of my own testing, initially also with a level 30 character. It seemed to me at the time that a value of 100% Thorns would deal 35 damage, but I also found that different enemies would not only reflect different types of damage, but also take different amounts based on their own defense values. Link → Thorns Damage Capped?
I found that the damage ranged from 27 to 47, depending on the enemy taking the reflected damage. I later tested a level 1 character and found that at 100% Thorns approximately 16.6 damage would be reflected. What I really found interesting was that when I changed to a world with a lower danger level on the same level 1 character wearing the same armor, the reflected damage dropped by over a third.
This implies that while character level plays a factor, the world’s danger level somehow plays in as well. It is also possible that the level of the gear with the thorns value might affect things, although that would require significantly more testing. Since I only have the resources I have the time to farm myself, it would be unreasonable for me to do all the testing myself.
Either way, Your Level is not the only factor. There is certainly more at play than meets the eye.