In Stock Rover we always use the close price from the day it is sold, so you just enter the quantity and the date when entering a sell.
We have a limitation where we can’t capture the gains made from day-trading (holding a position for less than a day) or from selling at a better price than the closing price. The analytics still tend to be very accurate as missed gains or losses tend to average out, close but not perfect. The brokerage integration services just provide daily snapshots of a portfolios holdings and our software has been designed around that convenient use case.
