You have to monitor peak utilization of the network on the order of seconds to minutes. The rule of thumb from queuing theory is that network performance tends to tank when capacity utilization hits about 80% => a 1 Gbps link is only good if your max utilization is 800 Mbps. But if your monitoring only looks at 1 hour averages over the course of a month, you're going to miss the periods of seconds where everyone's getting ping times of >100ms and having their VoIP calls drop out.

There are a lot of things you'll learn along the way if you try to run an ISP. =-)