I've found that the nominal weight and actual weight of light silks seem to be pretty close, when I've ordered silk.
But spinnaker fabric will always weigh about twice the advertised weight, in my experience. Some vendors, like Thruhiker, weigh it themselves and report the real weight, but the typical 0.5 oz spinnaker (including the sailrite stuff) usually weighs between 1.0 and 1.9 oz/yd. It has nothing to do with the coating or a conversion from sailmaker's yards. It's just conventional for sail and kite fabrics in the 1-2 oz range to be called 0.5 oz or 0.75 oz. I've asked vendors to weight it for me before I buy, and some have been shocked to find that their own "0.5 oz" spinnaker weighs 1.7oz/yard.