You should generally keep your core temperatures below 85 degrees celsius. As for voltage, I'd say set it to static, run at the base stock voltage it gives you and start moving the multiplier up in increments of 1. Then stress test with something like Realbench or Prime95 with AVX instructions off. If you encounter instability eg. stress test crashing or bluescreens then it means you need more voltage. If your temps exceed 85 celsius, then you likely need to improve your CPU cooling if you want to push more voltage. One thing to keep in mind is that by default the all core Turbo clock for the 8600k is 4.1GHz, so any multiplier below 41 is going to actually be slower than not overclocking at all, so I'd say start at a multiplier of 42...