This blogpost is focused on mutual funds. Here’s the FSM methodology

• Sound byte: when you buy, WAC may change, but not (never) when you sell. No FIFO rule.
• Sound byte: methodology was presumably designed for buy-once-never-sell, or buy-once-sell-repeatedly.

Before you sell, the WAC [weighted avg cost] was last updated in the previous buy event. From then on, all units are treated as bought at the same cost i.e. the WAC. When you sell 11 units,

1. units reduce by 11
2. investment amount reduce by … ? { 11 * sales price } I guess the updated value may approach $0
3. $ pnL i.e. unrealized PnL is updated to { units * WAC – investment amount }
4. % pnl = $ pnl / investment amount

$ profit is unrealized profit. (The realized profit is now show in the blotter.)

For stocks, the $10.30 upfront is part of the displayed cost

==== wife’s account end-to-end PnL before my 50th birthday