Fairest way to evaluate stock picks?

Currently I'm tracking all my purchases against theoretical equivalent purchases into an S&P 500 index – where the cost basis, purchase date, and eventual sell date is matched. My problem is that this evaluation method gives a big advantage to my “personal portfolio” as usually my buys/sells are performed at opportune times, when the price is right. And ignores opportunity cost. My purchases are not done with regular amounts or at regular intervals, so i'm not sure how I would compare it to something like a dollar-cost averaging strategy. How do you do it?