Why is my gui spazzing out instead of going to a 16:9 ratio?

I have a script (local) that changed the player's main gui size to fit the 16:9 ratio. BUT, for some reason it goes crazy and keeps going between two sizes and won't stop.

Here's the script:

local width = 16
local height = 9
local parent = script.Parent.Main

local running = false
local storedSize = parent.AbsoluteSize

local rs = game:GetService("RunService")

rs.RenderStepped:connect(function ()
	if running then return end
	storedSize = parent.AbsoluteSize
	if checkSize() then return end
	running = true
	parent.Size = UDim2.new(
		1,
		-(storedSize.X%width),
		1,
		-(storedSize.Y%height))
	running = false
end)

function checkSize()
	if storedSize.X%width ~= 0 or storedSize.Y%height ~= 0 then
		return false
	else
		return true
	end
end

I may have to do with how I make it get to the 16:9 ratio....

The math here doesn't make any sense.

% is remainder after division. Why are you taking the remainder after division by 9 and making sure that's not 0? That's 88% of Y values. For the % 16, that's 94%.

Just set the height to 9/16 times the width, or the width to 16/9 times the height, or each to some middle ground:

-- Just ignore height and resize vertically based on width
rs.RenderStepped:connect(function()
	local width = storedSize.X
	local height = width * 9 / 16
	parent.Size = UDim2.new(0, width, 0, height)
end)

-- Do some sort of in-the-middle of width and height
rs.RenderStepped:connect(function()
	local size = parent.AbsoluteSize
	local mw = size.x / 16
	local mh = size.y / 9
	local m = (mw + mh) / 2
	parent.Size = UDim2.new(0, m * 16, 0, m * 9)
end)