What is link Utilization factor?
It is simply the amount that the link is being used. It you have an ideal link datarate of 1 Gbps (Giga bits per second) but can only get 800 Mbps then your channel utilization is 0.8 or 80%.

Utilization expressed as a formulae will change according to where you are in the protocol stack. At the physical layer it will depend upon your modulation scheme, if you use 8b/10b then it will be close to 80%, the trend is towards higher ratio's like 128b/130b with utilization of 98.5% +.

In a higher layer you will be dealing with packet overhead, error correction codes, preambles and signalling bits.  In this case the utilization will be the \$ \dfrac{data payload}{data payload + overhead} \$.  You don't say what your scheme but all standard schemes have published data.