It can be left off, as long as you make sure to note and correct the units in the n term. If you leave n in the traditional SI units, k is just the dimensional analysis to convert to English. k = 1 for SI units, and k = 1.49 for English units. (Note: (1 m)1/3/s = (3.2808399 ft)1/3/s = 1.4859 ft1/3/s)