In extra, boinc preference, processor, "on multiprocessor systems, use at most", the number of CPUs would be easier than the percentage. I have two machines with 24 cores each. Converting to a percentage is a nuisance and requires me to get my calculator out!
Be aware that if this has to be converted to a percentage to give to boinc, then it should increase it slightly. Eg if I have 3 cores and enter 66.6%, it will use only 1 core. If I enter 66.7% it will use 2 cores. Boinc rounds down, not up.