At HiddenLevers, we use Adjusted Closing Price for all total return calculations seen on the site. The definition of Adjusted Closing Price:
An adjusted closing price is a stock's closing price on any given day of trading that has been amended to include any distributions and corporate actions that occurred at any time prior to the next day's open. The adjusted closing price is often used when examining historical returns or performing a detailed analysis on historical returns.
In terms of dividends, HiddenLevers includes dividends in the price history of any publicly traded investment, but does not assume reinvestment. This is in line with Adjusted Closing Price:
Common distributions that affect a stock's price include cash dividends and stock dividends. The difference between cash dividends and stock dividends is shareholders are entitled to a predetermined price per share and additional shares, respectively. For example, assume a company declared a $1 cash dividend and is trading at $51 per share on the ex-dividend date. On the ex-dividend date, the stock price is reduced by $1 and the adjusted closing price is $50.
See the Investopedia definition of Adjusted Closing Price HERE for more.