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)

One way do keep a GUI in the middle of a screen no matter the device is to set AnchorPoint to 0.5, 0.5 and Position should be set to {0.5, 0},{0.5, 0} this way centers your GUI in every devices screen no matter the size of the device. Hope this helps!