I don't know the original source, but it fairly common financial advice. Here is one source [1] but a simple DuckDuckGo search or Google Search will show many o

I don't know the original source, but it fairly common financial advice. Here is one source [1] but a simple DuckDuckGo search or Google Search will show many others. Some are starting to recommended 3-4x due to low interest which as I said in other comments I do not agree with. [1] https://www.investopedia.com/articles/pf/05/030905.asp https://www.investopedia.com/articles/pf/05/030905.asp