The fields being used are the bandwidth and traffic fields. The receive traffic is divided by the bandwidth to get the percent utilization. If the number is a decimal, then of it's .4 or below, it rounds down. The receive and transmit bandwidth are pulled from the speed value that the device reports to NPM. Yes, it is possible to have a different bandwidth than speed. If you have a 100Mb pipe, but you're only allocated 1.54Mbps, then you would edit the interface and set the receive and transmit to 1.54Mb. This would then allow the calculation to be done against what has been allocated from the port for your traffic. This is usually the case when the interface is provided by someone else. SNMP may discover the true speed of the pipe as 100Mbps which then shows as speed. But if you are only allocated a T1 on that pipe, then you'd want to set that custom bandwidth to change the receive and transmit values to the amount you are allocated, and show bandwidth utilization based on that.