If you didn't know, that 80% number is probably the result of Little's Law. That's the result where if your demand is generated by a Poisson process, and your service has a queue, 80% utilization of the service is where the probability of an infinite queue starts to get really high. People
Here's a nice blog post about the subject:
https://www.johndcook.com/blog/2009/01/30/server-utilization...