the difference between the 468 number and the 492 number, is that to get the correct length, you need to know the velocity factor of the wire or cable being used. with regular copper wire, the velocity factor is 0.95. with some transmission line elements the velocity factor can be as low as 0.6. 468/f is the wavelength in free space. 492/f is the wavelength on a wire dipole (as the result is 95% of 468/f)
468/f is often used in textbooks for antenna length because: a) it's easier to remember, and b) you can trim wire off an antenna to tune it a lot easier than adding wire to one that's too short