Actually the reflectivity of glass is only 5 times dimmer. I've made a number of flats using this test and no it's not as nice as using a reference flat in air. However if you lack a reference flat I think it is easier than making 3 flats for the 3 flat method IMHO. With the right light source, collimating lens and set up it is a useful test and kind of cool. It's biggest drawback is finding the required collimating lens but then Dick used a mirror for his collimator.