KEENO,
The reason your equation works is because it is the same as the one in the CAP698. 0.987 is approximatley 6000/6080.
And 101.3 (ft/min per Knot) is approximately 1/0.987, which is why all three methods work. They are all just different versions of the same equation.
But you are correct, yours (appears to be) easier.