What I'm trying to say: never apply Serpent Sting to a single target via Multishot.
Why? Multishot with ToTH costs 20 focus, does imp SSting damage, a physical damage and applies dot. (and consumes a chardge of ToTH)
Applying SSting manually costs 15 focus, does imp SSting damage, and applies dot.
eliminating the stuff both do equally, results in:
Multishot: does physical damage
SSting manual: save 5 focus, saves a charge of ToTH
As the direct damage of multishot is so weak, you do more damage by reapplying SSting manually, and using that ToTH charge on an Arcane shot. It's as easy as that.
(if you don't even have a ToTH charge, you are paying 25 focus for the physical damage the multishot does. it's just stupid, as that focus invested into an arcane shot is much more damage. more than double the damage)