For in game purposes. So would King's Rock + Air Slash + Serene Grace = 70% chance of flinching or 80% chance of flinching? (i.e. does Serene Grace double the King's Rock flinch rate?) Let me know! Thanks!
King's Rock doesn't increase the flinch rate of moves which already have a flinch rate- it just adds a 10% flinch rate to moves which don't have a base chance to flinch. It is boosted to 20% by Serene Grace, though, and each hit of a multihit move can proc King's Rock (which is why it's a decent item for Skill Link Cloyster).
A Serene Grace Togekiss holding a King's Rock and using Air Slash would still only have a 60% flinch chance.