Most SPL meters have a short (sometimes long) shield around the mic that makes it more directional to limit the capture to a small'ish area (volume). Unlike many measurement mics, which typically have broad response (omni's are commonly used), an SPL meter is generally used to measure a specific target area. It should work fine for headphones, at least it did for me way back when. At the time, I compared my approach to a head (bust) mounted unit the university's audiology lab had, and for levels it was basically an exact match. Frequency response differed somewhat since my rig measured essentially "flat" response (I did not have an HTF* model).
FWIWFM - Don
* HTF = Head-related Transfer Function, see e.g.
https://en.wikipedia.org/wiki/Head-related_transfer_function
@Quinton595 : You should be able to edit the title in your first post, or click on the "Report" link at the lower left of that post and ask a moderator to do it, and add the word "headphone" in there for future reference.