What should the CPU usage be of a fully-loaded CPU that has been throttled?

100%, which is what Windows Task Manager reports. Anything else is misleading because "potentially available" is vague especially with self-overclocking CPUs that rarely reach their full speed (and you can still overclock them beyond that manually, if you try hard enough... so where is the limit?) His argument for the 2nd option is moot when recent versions of taskmgr also report the CPU frequency.