Make r15 arm follow mouse?

Hello i'm trying to figure out how i can make the r15 arm follow the mouse here's my attempt

local char = script.Parent

local m = game.Players.LocalPlayer:GetMouse()
local Neck = char:WaitForChild("RightUpperArm"):WaitForChild("RightShoulder")
local NeckC0 = Neck.C0

m.Move:Connect(function()
Neck.C0 = NeckC0 * CFrame.Angles(math.asin(m.Hit.LookVector.Y),0,0)
end)

This work's it's just it isn't accurate like i put my mouse to the middle of the screen and my arm is pointing down? My guess is it's the lookvector maybe help please!