If I buy BTC at $1,200 and then sell it at $1,500 it is clear that I have made a profit of $300 minus the exchange fee. If I keep buying BTC at the same price over and over, and the price keeps increasing, I can measure all my gains in a similar way: just subtracting the current rate minus the price I have bought the BTC at. However, how can I measure profit or losses when buying BTC at different prices?
For example, say I bought 1 BTC at $1,200 and then 1 BTC at $1,500. If I sell 1 of these 2 BTC at $2,000, what buying price should I measure this sell against, $1,200 or $1,500, to calculate my profit. Maybe an average or weighted average of the prices? Am I missing something basic?