How Can I Keep a GUI In a Constant Position Despite Screen Size?

I don't understand AnchorPoint or Position well enough to figure out how to get a GUI to remain in a constant relative position despite my screen size. I am trying to make an inventory GUI that tweens out from the left edge of the screen (meaning most of it shouldn't be visible when its not opened); however, when I stretch my window to a different size, the part of the GUI that should be hidden beyond the left edge becomes visible. How would I go about fixing this? Here is my code if that means anything:

local TweenService = game:GetService("TweenService")
local inventory = script.Parent.Parent

function onClicked(GUI)
    local openGoal = {}
    openGoal.Position = UDim2.new(0.02, 0, .5, 0)
    local closeGoal = {}
    closeGoal.Position = UDim2.new(-0.1, 0, .5, 0)

    local tweenInfo = TweenInfo.new(2)
    local openInventory = TweenService:Create(inventory, tweenInfo, openGoal)
    local closeInventory = TweenService:Create(inventory, tweenInfo, closeGoal)

    local curr_pos = inventory.Position

    if curr_pos.X.Scale <= -.09 then
        openInventory:Play()
    else if curr_pos.X.Scale >= 0.019 then
        closeInventory:Play()
    end
    end
end
script.Parent.MouseButton1Click:connect(onClicked)