Why Velocity factor (VF) is simply the speed of the wave travelling in the line expressed as a fraction of c, the speed of light in free space: VF = v/c. Because the dielectric surrounding the conductors slows propagation, VF is always less than 1, roughly 0.66 for solid polyethylene coax, about 0.8 for foam coax, and 0.95 or more for open wire line. VF also equals 1/sqrt(dielectric constant), and you use it to find physical length: physical length = VF times the free space length.
Watch out The multiplication version would give a meaningless number far larger than c; velocity factor must be a ratio less than one, so the wave speed has to be on top of the fraction. The impedance ratio choices describe standing wave ratio, not velocity factor.
VF = line speed divided by c, always under 1. Coax 0.66, foam 0.8, open wire 0.95.