What is Reddit's opinion of Reed Instruments R8090 (SC-05) Sound Level Calibrator for 1/2" Diameter Microphones, +/-0.5dB Accuracy?

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Any decibel meter will measure 30db or so, because the cheap ones are calibrated to 30db in a quiet room. Decibels being logarithmic, cheap meters get very inaccurate very quickly. For instance, most of the cheap ones I've used max out at measuring 70-75db. And that's probably what's happening here, the magnet in the headphone driver is interfering with the microphone in your meter and it's getting stuck at reporting the maximum value it's capable of reporting. This is all especially bad, because when working properly, you could be pushing well, well into hearing loss territory, 90-100db and beyond, and they'll just keep reading 70-something db and make it look safe. OSHA-certified ones are calibrated at around 100db, and should be calibrated before each use, using something like this: https://smile.amazon.com/Instruments-R8090-Calibrator-Diameter-Microphones/dp/B008S0OVR2

So you can try solving your problem, but even if your meter was working 100% as engineered and you depended on it for accurate readings of headphone pressure, you're going to end up damaging your hearing, because it will most likely grossly undermeasure what are very dangerous sound pressures.

I assume your ultimate goal is to protect your hearing, and fortunately it's easier than taking measurements with questionable equipment. Listen to your headphones at your preferred fun volume for 10 minutes, and then sit in silence for 5 minutes. After 5 minutes of silence, continue listening to music at the previous volume you were using. If it's uncomfortably loud after sitting in silence, it's dangerously loud. Repeat this process until you find your maximum volume that isn't uncomfortable after sitting in silence and so long as you don't push past that, or at least not very much for very long, then your hearing will last a lifetime.

cross spectrum labs offers calibration services https://csacoustics.com/services/products/ for $55, and will sell pre calibrated mics as well.

EDIT: my bad, completely missed SPL in the thread title.

a quick and dirty way is to compare to a know good meter, ie play a know signal at a fixed volume and distance and take a reading with the know good meter, record the results then replace the know good meter with your mic at the same positio and distance, and copmpare your reading, apply offset to your results to match the know good meter. this is by no means perfect, or super accurate, but if you have more time than money its better than nothing, but not by much.

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cheaper calibrator, but i don't think its NIST tradable https://www.bhphotovideo.com/c/product/462916-REG/Galaxy_Audio_CM_C200_CM_C200_Calibrator_for_SPL.html/?ap=y&gclid=CjwKCAjwycfkBRAFEiwAnLX5ISYi0pumdupkgMOtymnm3JoamuO980DIshHmOYc8h1Bu4kTkiOe5NxoC9RgQAvD_BwE&lsft=BI%3A514&smp=Y

even cheaper, not nist tracable https://www.amazon.com/dp/B008S0OVR2/ref=sspa_dk_detail_2?psc=1&pd_rd_i=B008S0OVR2&pd_rd_w=Y1WTx&pf_rd_p=733540df-430d-45cd-9525-21bc15b0e6cc&pd_rd_wg=JSgPY&pf_rd_r=E1EABSK0SAD0QSKA2RMC&pd_rd_r=7bc70afe-4b36-11e9-8f27-b16cce697188