If you hover over to the following & click while in auto,
- CPU vore voltage override
You should then be able to enter a manual vcore number i.e. 1.325V in the field - set the manual clock figure by pressing enter before then saving & exiting the BIOS.
Sometimes while stress testing the vcore itself will fluctuate slightly based on the type of load / stress being put on the CPU. Obviously if you're set to auto then the motherboard will pump what ever volts it thinks is required for the load on the CPU at any given time. However as you set it to a manual vcore of i.e. 1.325V, you may find that CPU-Z will report during stress testing that the vcore can bounce anywhere between 1.292V-1.350V. This is somewhat normal depending on the load but this can also be tightened up by then delving into the LLC (Load Line Calibration) settings.
On my setup which is similar to yours (Asus Z170I Pro Gaming & 6600K), I'm at 4.5Ghz @ 1.250V & everything else set to auto - I didn't feel the need to adjust my LLC settings as it's still stable & the vcore only ranges between 1.248V - 1.250V while stress testing.
Liam.