math, maybe it means if yer using PNU (approx 8.2 - 8.6 ft, dep. on poser vers.), ya gotta lower raybias proportionately.  ISTR one of 'em mentioned that above.  the manual doesn't specifically say that raybias is function of user's units pref, but what if ya changed yer units to inches or feet or metric unit? if changed to inches, maybe raybias could be 100X larger.