IMO, 97% should be alright on your system, provided that the FahCore_22 tasks is scheduled at sightly higher priority than the Rosetta tasks.
I don't know if it's still the same with Windows 10, but IIRC Windows 7 had 5 scheduling priorities: The normal, two higher, and two lower. BOINC CPU work is running at least priority by default, and FahCore_22 should ideally run at the 2nd lowest priority. That way, as soon as the system needs to run something else, some of the Rosetta tasks get preempted before the FahCore task.
Can somebody with a Windows 10 system confirm at which scheduling priority FahCore is running by default?