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()