assume fl8 is f/8
150" at f/8 means the focal length = 150*8 = 1200 inches
divide by eyepiece of 1/2", the magnifying power = 1200/(1/2) = 2400X
150 inches at f/8 gives a focal length of 1200 inches.
Divide 1200 inches by 1/2 inch eye piece to get the magnification of 2400x.