I don't think this makes much sense. The maximum throughput of a CPU in terms of instructions depends on so many factors (thermal, memory, instruction mix) that

I don't think this makes much sense. The maximum throughput of a CPU in terms of instructions depends on so many factors (thermal, memory, instruction mix) that trying to summarize all of them into a linearly-scaling "utilization" metric is a bit tricky. You can measure the fraction of time CPU cores are busy, the number of instructions executed, frequencies, etc. to get an idea, but only experimentation will tell you what "100%" of a system's capacity is.