This is how I calculate and also got same as yours:

((NrofPayouts * DailyPayout) / DelegatedSP) / ((( NrofPayouts + Cooldown) / DaysInWeek) / WeeksInYear)

((7 * 0.129) / 150) / (((7 + 5) / 7 / 52) = 0.006 / 0.033 = 0.182 (18.2%)

Thanks for letting us know!