Calibrate Graphic EQ without SPL Meter
If you know the curve for the mic you're using, then you should at least be able to determine what response you have now, which would tell you if you need to make EQ changes. Doing it band by band might be a bit tedious, but you won't do it often, so...

The gold standard is something like an Ivie sound analyzer, which uses "pink noise" as the sound source, and one or more calibrated microphones as pickups. You can monitor the acoustic properties of the room, from different positions, and see changes in EQ reflected in real time across the entire spectrum.

They're too expensive for casual use, but you may be able to rent one, or hire someone who owns one for a nominal fee.