Further to my post above, the advanced voltage area of my bios strangely reports a 1.100v vCore to which I add 0.054v offset to get a reported 1.248v on the bios main status screen ??. Perhaps I am fighting against the chip's dynamic voltage controller.
Ultimately though, this setting has pegged the maximum voltage to 1.248v. Gigabyte's System Info Viewer reports a vCore of 1.2 which drops to 1.88v at maximum 8 thread load and 4200mHz speed.
Confusing stuff. But this setting knocks about 20C off the temps, so I am happy.