When adding to, scaling out of or adjusting positions, it is normally difficult to keep track of P&L. However Thinkorswim has an indispensible feature for keeping track of positions using Simulated Trades, and exporting/importing the position history.

If you keep your exported trades in a cloud folder like Dropbox (highly recommended!), you can access these files from multiple computers, and you have the added benefit of automatic backup and previous file history.

Make sure to set the viewing quality to high resolution on YouTube.