How to properly use magnitude?

This script is supposed to increase if you get farther away from the brick, ad decrease as you approach it. Just so you can see it wherever you go. My problem is that it keeps increasing and decreasing randomly. Tips?

local Sizing = coroutine.wrap(function()
	pl = game.Players.LocalPlayer
	c = game.Workspace.CurrentCamera
	s = c.Part
	f = s.Rank.Frame
	old = 0
	while true do
		wait()
		if (old < (pl.Character.Head.Position - s.Position).magnitude) then
			f.Size = f.Size - UDim2.new(.1,0,.1,0)
			old = (pl.Character.Head.Position - s.Position).magnitude
			wait()
		elseif (old > (pl.Character.Head.Position - s.Position).magnitude) then
			wait()
			f.Size = f.Size + UDim2.new(.1,0,.1,0)
			old = (pl.Character.Head.Position - s.Position).magnitude
		elseif (old == (pl.Character.Head.Position - s.Position).magnitude) then
			wait()
        end
    end
end)
        game.Lighting.Part:clone().Parent = game.Workspace.CurrentCamera
        Sizing()