A helpful resource for voltage/frequencies for cpu's is the "Silicone Lottery Historical Binning Statistics". Silicone Lottery bin chips for their overclocking capability and then sell them for a fee above the msrp. They use a minimum 240mm water cooler. They list 1.375v for 4.8 all Core, non AVX. From what I've heard for Coffee Lake, chips are @1.20ish v for stock voltages. The voltage a particular cpu 'needs' are based on the stability parameters you've set while maintaining safe temps. The less voltage for stability for a given overclock the better. I always try to apply the minimum voltage for a given frequency and stability for even better temps though your voltages and temps are all within acceptable ranges.