Whats wrong with my healthbar?

wait(.5)
game:GetService("StarterGui"):SetCoreGuiEnabled("Health", false)
 plr = game.Players.LocalPlayer 
 chr = plr.Character
 hum = chr.Humanoid
 sp = script.Parent
 bar = sp.Bar
 hbar = bar.Health

while wait(.001) do
    hbar.Size = UDim2.new(hum.Health/hum.MaxHealth - 0.368, 0, 0, 35)
    if hum.Health<=20 then
        hbar.BackgroundColor3=Color3.new(255/255, 0, 0)
    else hbar.BackgroundColor3=Color3.new(0, 255/255, 0)
    end
end

hubar = bar.Hunger

This local script controls the health bar, This is the layout:

layout

It works well but the length of the healthbar isnt equal to my health. Like whenever my health is 10 the healthbar falls into negative numbers and looks like this:

Health Bar problem

How do I stop this from happening? (the original length of the health bar is {0, 700},{0, 35})