Setting your monitoring levels to 86dB SPL using S4X headphones requires some basic math and understanding of sensitivity information. Yes, we think 86 is a better option than 83 when it comes to headphones.
We agree with Bob Katz's K metering proposal available in AES library, and since the headphones produce two separated signals, we believe it's better to have 86dB SPL in each cup than 83dB SPL that is recommended by the K Metering.
In this video you'll see the "hands on" process of setting that up by using RME interface. Of course, the process is the same with other proper audio interfaces.
You can find resources on the links below: