Vertical UI Slider not working properly?

Hi, I'm trying to make a vertical UI slider, and it almost works but the slider button slightly shifts to the right every step. Code below v v v

local Slider = script.Parent
local SliderBtn = Slider.Button
local Player = game:GetService("Players").LocalPlayer
local UIS = game:GetService("UserInputService")
local RuS = game:GetService("RunService")

-- Properties
local held = false
local step = 0.2
local percentage = 0

function snap(number, factor)
	if factor == 0 then
		return number
	else
		return math.floor(number/factor+0.5)*factor
	end
end

UIS.InputEnded:connect(function(input, processed)
	if input.UserInputType == Enum.UserInputType.MouseButton1 then
		held = false
	end
end)

script.Parent.Button.TextButton.MouseButton1Down:Connect(function()
	held = true
end)

RuS.RenderStepped:connect(function(delta)
	if held then
		local MousePos = UIS:GetMouseLocation().Y
		local BtnPos = SliderBtn.Position
		local SliderSize = Slider.AbsoluteSize.Y
		local SliderPos = Slider.AbsolutePosition.Y
		local pos = snap((MousePos-SliderPos)/SliderSize,step)
		percentage = math.clamp(pos,0,1)
		SliderBtn.Position = UDim2.new(percentage,0,BtnPos.X.Scale, BtnPos.X.Offset)
	end
end)