How do I calculate profit and loss (P/L), mark-to-market method?

Mark-to-Market Valuation Explained

The mark-to-market method values positions based on their current market price, providing an up-to-date reflection of their worth.

Let us assume in March there was a purchase of 50 units of stock XYZ at a price of GBP 15 each, and a commission of 2%. The total cost of the transaction is GBP 750 plus the GBP 15 commission. The total costs incurred are calculated as:

At the end of the day, the XYZ stock price has increased to GBP 16. The current market value of the position for XYZ is:

Using the mark-to-market valuation method, the Instrument P/L for stock XYZ on Day 1 is:

The following day, the market price for XYZ stock has increased to GBP 16.50 by the end of the day. The end-of-day current value (Position Value), at the end of Day 2 is:

And given that the Position Value at the end of Day 1 was GBP 800, and this is the opening Position Value used at the start of Day 2. The Position Value at the end of Day 2 is GBP 825. The profit/loss for stock XYZ, using the mark-to-market valuation is calculated as: