The blog

Around 85dB SPL is considered sort of the “industry standard” listening level, but that’s historically been a rule of thumb for studios with huge monitors and control rooms that are large enough to handle those kinds of SPL’s without natural room compression, and monitors that can handle them without breaking or distorting.

The standard for near-field monitors is around 73dB SPL.

For more in-depth info on the “why,” search up the Fletcher Munson Curve.

Monitor level calibration tutorial: https://www.production-expert.com/home-page/2018/10/2/monitor-level-calibration-how-to-undertake-a-stop-gap-monitor-calibration-that-doesnt-trash-your-pro-tools-session

 Gearspace link discussing: https://gearspace.com/board/mastering-forum/1024714-depth-bob-katz-k-system-tutorial.html

SPL METER = sound pressure level meter.  Phone app versions work, but require a meter to calibrate. So just buy one.

another fletcher munson curve comment:

It’s got to do with Fletcher Munson curve, which gives you a “flattish” response at 85db, especially for lower notes and sub bass, which needs air pressure to be heard properly. Lotta folks work 60-75db while writing, and crank up 85 for checking the mix.

I use an app called decibel x on iOS to get rough measurements while working.