To calculate mutual fund average cost do I use only shares bought before the sale?

In 2019 I sold some of the shares of a mutual fund that I own, but now all the shares. I need to calculate the average cost per share.

Am I correct that I only use the total cost (from automatic reinvestments from dividends) for the period up to the date of the sale? And the divide that by the number of shares owned on that dae?

Or, instead, do I use, the total cost for the period up to the present day? And then divide by the number of shares owned at present?

(As this is a fund in which I owned shares for many years, it is not "covered", hence the broker does not provide the basis on 1099-B.)