Not wishing to be pedantic but shouldn't
divide GS by 2 an multiply by 100
be
divide GS by 2 and multiply by 10.
using
divide GS by 2 and multiply by 100
gives
GS 150, so it will be 650 / 2 x 100 = 7500 fpm
GS 200, 10000 fpm
GS 130, 6500 fpm
But the method is far better than many that I have seen.