I prefer to think in nm/min. You can just plug nm/min into any speed based rule of thumb, I find it much easier to juggle 1.5,2,2.5,3 etc in my head than thinking in longhand (15/150*60).
That 150 kts. Is that TAS or IAS? If it is TAS then it should be the the average TAS from 8000' to 1000', who would fly a constant TAS when descending? Assuming it is IAS it doesn't change it much, rule of thumb answer using the rule of squares for TAS still pops the answer out at ~1200fpm. However, will make a big difference if descending from hypoxic altitudes though.