Use a finishing hydrometer to get the most accurate reading possible. SMV is a subjective preference, but a terminal gravity of 1.000 is generally a good place to start for any sake. Nigorizake tends to be slightly higher, in the 1.002 - 1.005 range, to counteract the bitterness imparted by the koji particles left behind suspended in the cloudy sake.
Refractometers are useless for sake, with or without correcting for the presence of alcohol, unfortunately. It has to do with there being no OG to compare the FG against and the high amount of suspended rice particles in the solution.
Sake, being made from grain, could be considered a type of beer. :cheers: