How is your 5D3 with other lenses? From a personal standpoint that seems excessive, negative or positive at that amount. Being a prime and if you are happy with it then it is up to you. I would send it to Canon, but that is me. I'm not saying you should.
After fiddling with the light and target, and following the instructions to the letter, my adjustment is +7 . On my 24-70 it is +4 at both ends and the 70-200 is a +5 at both ends. And, low and behold, the pictures are sharper.