You'd want it a bit less than that for headphones.
I found this method worked quite well, and agreed with the speakers quite well when set up correctly;
(blog) Measuring the SPL volume (dB) of headphones
Obviously there is some leakage and so on, so allow for that, and also be awre that you do want it several dB lower for cans.
But all told it helps to measure it all so you have repeatable headphone levels, IMO and IME.