Why this grid placement system code only works on specific baseplate size?

So I made this grid placement system thingy and the block keep floating, so i decided to change the baseplate size and then it somehow works

The code:

local Player = game.Players.LocalPlayer
local plrChar = Player.Character
local mouse = Player:GetMouse()

function SetUp()
	local NewBlock =  script.Part:Clone()
	NewBlock.Parent = workspace
	NewBlock.Transparency = 0.5
	NewBlock.CanCollide = false
	return NewBlock
end

local Thing = SetUp()

function Round(Input, GridSize) 
	local X = math.floor(Input.X / GridSize + 0.5) * GridSize 
	local Y = math.floor(Input.Y / GridSize + 0.5) * GridSize 
	local Z = math.floor(Input.Z / GridSize + 0.5) * GridSize 

	return Vector3.new(X, Y, Z)

end

mouse.Button1Down:Connect(function()
	Thing.CanCollide = true
	Thing.Transparency = 0
	Thing = SetUp()
end)

game:GetService("RunService").RenderStepped:Connect(function()
	Move()
end)


function Move()
	local target = mouse.Hit.p
	local face = mouse.TargetSurface
	mouse.TargetFilter = Thing
	local norm = Vector3.FromNormalId(face)
	Thing.Position = Round(target + (norm * 2), 4)
end

Hmm, I'm not completely sure but I'd guess it's something to do with all of the rounding. How big does your baseplate have to be for it to work? And at what size does it stop working?