As far as I'm aware, you can't determine the actual covalent radii values simply from that information. The best you can do is to assume equal change in size for each ion upon atom formation and express it as a formula:
r(K) = 1.34 + x
r(F) = 1.34 - x
looking up the atomic radii for both of those elements shows that x ~ 0.63 Å. This was a lucky assumption since choosing any other metal would have yielded a considerable different value.