What should my voltage be for my i7 4790k if it's going to be overclocked to 5.0 ghz

Other than every CPU being variable for overclocking, for 5 ghz you're putting the cart before the horse as to whether you can even stabilize it for anything under 1.40, with 1.30 volts (mine's 1.32 for 4.7) being the max recommended voltage to begin with. Haven't seen too many 5 ghz chips. Try 4.6-4.7 at 1.25 volts and scale upwards in both frequency and voltage as needed, 10 min OCCT should let you know if you're stable or not.