How to make a leaderboard to display large numbers?

By large numbers, I mean numbers like 239 quintillion or 676 septillion, etc. I know I'm probably going to have to use string values like "239Qn" but I don't know how to exactly get around determining whether if it's a thousand, a million, a billion, and so on, and once I do that, how do I determine if it's in the ones, tens, or hundreds. I've tried this:

local index = {4, 5, 6}

game.Players.PlayerAdded:connect(function(plr)
	local cash = Instance.new("StringValue", plr)
	cash.Name = "cash"
	cash.Value = "0"
	cash.Changed:connect(function()
		if cash.Value:len() == index[1 or 2 or 3] then
			cash.Value = cash.Value:sub(1, 3).."k"
		end
	end)
end)

But it prints all in the hundreds place, and I don't think it's efficient either.

Not a request, simply asking how to get around it.