Most analyses I've read say the threshold is around the 80% mark [1], although it depends on how model the distribution, and there's nothing magical about the number. The main thing is to avoid getting close to 100%, because wait times go up exponentially as you get closer to the max.
Little's Law is fundamental to queueing theory, but there's also the less well-known Kingman's formula, which incorporates variability of arrival rate and task size [2].
[1] https://www.johndcook.com/blog/2009/01/30/server-utilization...