Welcome. We have not heard from Stuart in some time, so I guess I could help you here.
dB or Sound Level Meters come in a very wide range of price and quality. Indeed you may still see mention of the original Radio Shack one, which was a truly awful POS. Over time things change. Now iPhones can become really decent fully featured SLM. The free and cheap ones are fine, I have tried most, but I do appreciate the more Pro nature of Faber SoundMeter and this also this company https://studiosixdigital.com/audio-hard ... phone.html
I use my iPhone all the time when doing safety surveys in clubs and venues. One wouldn't want to wander about on a busy saturday night with a Bruel and Kjaer or other big conspicuous mega buck meter.
If your Mic has a Cal file, then REW should be able to read it and become a 'free' SLM. The accuracy will be equal to that of the Cal file.
I have found the UMIK-1 to be 1.8dB off in two samples from different years. I have found iOS apps to be closer than that.
Comparative Calibration is very simple. REW, SoundMeter etc. can be calibrated against a trusted exterior SLM, or a dedicated Calibrator Device if using an external 1/2" Mic
You just play a steady Pink Noise, adjust the level to some convenient number on your borrowed, ideally Calibrated, but in any case trusted SLM. Say 75dB. Then use the Cal trimmer in your app to get it to read the same.