Dividend rein vestment question

[ngcpa]

I am trying to build a data base for experimentation purposes.  I have a question about dividend reinvestment.  What is the price used?  Is it the opening or closing price on the x-date or the payable date or what?  Morningstar lists the distribution NAV which seems worthless.  Thanks in advance.
Norm

[LifestyleFreedom]

I have dividends automatically reinvested in one of my online brokerage accounts.  Here is the fine print from a footnote about how this particular brokerage firm reinvests my dividends:

Cash dividends are generally paid on the payable date and reinvested the next business day. The price of the shares purchased by your dividends is based on an average share purchase price. Timing of purchases is subject to our discretion, although our system automatically places trades at 10:00 a.m. eastern time.