The focal point is the same for Ku or C. With prime focus dishes, the F/D ratio is typically .30 to .45. With offset dishes, the F/D ratio is typically around .6. The feedhorn is optimized to match the F/D ratio of the dish. If you use a feedhorn optimized for an offset dish, like a DBS LNBF, for a prime focus dish, the LNB waveguide will reject signal coming from the perimeter region of the dish, whereas if you use a prime focus styled LNBF for an offset dish, the feedhorn will "see" a little more off-boresignt noise and develop a slightly inferior signal to noise ratio.