Do a search on Youtube for Der8auer's 8700k overclocking guide, he did his on a Maximus X Hero, though do keep in mind that guide assumes you have delidded your chip, on a stock CPU it's probably not a good idea to immediately shoot up to 1.4 for Vcore. Still it includes some helpful info regarding CPU current and power limits which you'll need to disable to avoid throttling as you overclock.
4.7GHz is a decent starting point, it's the single core Turbo value. As for how much voltage you'll need, that will vary from chip to chip. You could start at about 1.25Vcore at 4.7 and test for stability and thermals. Most chips should be stable at 4.7GHz at 1.25V. If you are and your temps aren't in the 80s, then you can try 4.8 and increase voltage if necessary if you run into instability. 4.9GHz is usually doable without a delid, though things might get a tad warm, especially if you have to push past 1.3V to get stability.