Looks like the meter that Will sells at makeitlouder.com. I own one and will vouch for it being rugged and consistent. The caveat being you must have an accurate DMM to use it.... though you probably should own one anyway if you're at the point where you need an SPL meter.
Even simpler without even worrying about the conversion chart is to just look for gains or find peak in mV. Not like you need to know the precise dB number.... just just need to know your peak and whether you're losing or gaining.