Ways to keep track of and calculate cost basis

Hi, what's the standard way to calculate your stock's cost basis if you trade it from time to time? Should you include realised gain/loss in your weighted average price calculation?

For example, if I bought 100 shares and later sold 20 shares for profit, if I include this realised gain, my average price drops.

This question is geared more towards stock trading (vs stock holding), so the average price is quite important to determine when to stop-loss or take profit. Thanks!