Nope. It all depends on what you want to calculate. If you want a speed, you need to take a distance and divide it by the time, because that's how the word "speed" is defined:

vec_scale(Rot_Speed,1/time_frame);

But if you don't want an angle speed but just the angle difference between frames, the original calculation was fine. I don't know what he really needed, but assume it was indeed a speed.