I think if you are going to go the custom route, you have a few things to consider in order to compare the performance of your investments.
1. An investment that earns $100 profit on $500 is better than an investment which earns $100 on $5000. Size of initial investment matters, so all profits should be expressed in % form.
2. Earning $100 over one month is better than earning $100 over one year. You should report annualized gains rather than % gains over a specific period.
3. Earning USD$ is different from earning CAD$. Exchange rates fluctuate frequently and have an ongoing risk. Just because you maintain your portfolio in USD$, doesn't mean you haven't had any currency gain/loss. You should always express your current values and annualized % returns in the currency you will need if you were to cash in the account and spend it. Use any reasonable exchange rate to convert on the settlement date of the purchase/sale. As long as it is reasonable and consistent, you should be fine with whatever rate you choose.
4. Average cost basis is required for tax purposes, so you might as well get used to it. Track your investments by ticker symbol using the average price of all purchases as the cost basis for any sale. i.e. Buy 100 shares @ $10, Buy 100 more @ $15 = Average Cost of 200 shares @ 12.50.
5. Always consider the commissions both to buy and to eventually sell the stock as part of the cost basis (for the purposes of analysis only). Remember also to include a reasonable average tax rate on income earned to determine your real profits.
6. It gets a little complicated to determine a comparable rate of return for dividend payments due to the timing of payments and the different tax rates, but they have a cost basis (the average cost of all shares on which the dividend was calculated), and a clearly defined time period in which the income was earned (typically quarterly), which allows calculation of an annualized % return for each dividend payment. The value of the dividend payment should be considered to have been received in cash, and the value of any reinvestment should be considered as a separate purchase adding to the average cost on which future dividends are calculated.
7. The annualized % return does not typically consider idle cash (opportunity cost). Therefore, you could earn $1200 on a $10000 investment in 30 days and make no other investments for the rest of the year and still consider your annual 144%, but really you should consider 0% earned on the idle cash expressed as an annual return for the remainder of the year and net that against your returns.
8. The annualized % return can be easily skewed by short term investments. $50 earned on a $10 investment over 5 days is not as important to your total return as $5000 earned on a $1000 investment over 5 days. If you are comparing investments individually, this is not a problem, but to make average returns for the year for all investments, you will need to come up with a reasonable method of weighting the returns.
8. The annual % return does not consider risk. One investment may have a 90% chance of $100 loss and 10% chance of $100 gain, while another may have 10% chance of $100 loss and 90% chance of $100 gain. Just because they both happen to earn $100, they will have the same annual % gain (all other things equal). But to use this information to learn and to make similar investments in the future, such risk analysis should be considered.
As you can see, there is a lot of room to customize the performance tracking to your specific needs. At each stage, you should ask yourself what it is you really want to track.