mellored
Legend
3d10 * 50% chance to fail the saveNot following your calculations
+3d10 * 50% chance to deal 50% damage.
= 12.375
Did the same for Lightning Arrow and compared the difference.
Treantmonk uses 16 rounds of combat per day (4 combats * 4 rounds), and divides by 16 to get an average damage per round. Boost Wis to for 60% rate at higher levels, and he uses half the spell slots for damage (level 3, 4, 4, 5). Lower level slots are still Hail of Thorns, so I am just looking at the damage boost of these 4.
I didn't calculate the level 4 slot difference. But 1 is nearly between .77 and 1.2 so I just rounded to 1* 4.
Also. I didn't add Lightning Arrow on a crit.
...
But that's wrong. Lighting arrow doesn't add damage. It replaces damage.
Treantmonk is probably correct then.