How should I handle cone spells on grid?

@PhilippMordasheHow should I handle cone spells on grid? I’ve made triangle papercut for 15ft cone, but it contradicts diagonal movement rules I'd go with the paper, hitting any square 50% or more filled #WOTCstaff

— (((Mike Mearls))) (@mikemearls) August 15, 2016

@PhilippMordashewhat about squares with less than 50%? Are they hit but with advantage on saving throws or not hit at all? I'd run it as non hit at all, but save with advantage is a good compromise #wotcstaff

— (((Mike Mearls))) (@mikemearls) August 15, 2016