Disclaimer: I'm not an acoustics expert by any account; this is just to share some accurate sources of information I have found helpful, and revise the information for myself.

As the home studio has become increasingly central in carrying out professional work, calibrated monitors are necessary to ensure that work will translate accurately on different, larger systems. A mix that sounded nice and balanced on the home set up, is suddenly boomy and muddy on a larger system.

It all comes back to the Fletcher-Munson curve. At lower listening levels, we can't perceive bass and high frequencies very well. At around 80dB SPL, the curve flattens out and our perception of the frequency spectrum becomes more balanced. Meaning you won't be trying to compensate using EQ boosts, and unintentionally muddying your mix.

Sound on Sound have a great, free article on steps for calibrating. Note the table at the end that advises SPL calibration levels depending on room size. The 83dB SPL figure is often bandied around forums, even though it only applies to larger rooms with mid to far field monitors; it originates from Dolby calibration of dubbing stages. In a small room with near fields, 83dB SPL is deafening.

The SPLnFFT app came in handy as an iPhone SPL meter, and has according to research papers, come out as one of the more accurate SPL meter apps.

I recently re-calibrated my speakers in a new room to the 74dB SPL level which is appropriate to the cubic volume of the room. I also hung acoustic foam on the walls to help dampen the flutter echoes in the room, and have some bass traps in the the corners. It's still far from a perfect room, but treatment works exponentially - a little will go a long way. Acoustic treatment is vital if you're to benefit from monitor calibration, otherwise you'll still end up EQing incorrectly as you eliminate frequencies that are interacting with the room.