One restaurant that I've been to with friends when I visit them in California is in a touristy area. When they print their bill, they put right on it, across the bottom, what 15%, 18% and 20% would calculate out to. I know some people aren't crazy about the idea but I LOVE it, especially when I'm travelling. It lets me know what is considered an average tip in the area and I can then determine how much I want to leave. So if the service was poor but not abysmal, I can leave the 15% amount, rounded down to an even number when added to the bill amount. If the service was decent, I can leave the 18% amount, rounded to the nearest even number when added to the bill amount. If the service was stellar, I can leave the 20% amount or more, rounded up to the next even number when added to the bill amount.
In response to the OP's original question, I like to leave the tip in cash, if I have it. I don't always have loonies, twonies and fives on me. When that's the case, I put the tip on my credit card. I usually tip 15% on the total amount. It used to be 15% on the subtotal but it has steadily crept up here, too. It used to be really easy because our tax was 15% so 'tip the tax' was the standard. The tax is down to 13% but I still use that as my guide. So if my bill was $16.81 on the subtotal with $2.19 in tax for a total of $19.00, I'd add 2.19 and round up to the nearest $ so $22 total, which would be $0.15 over 15% on the total. And I'm glad I just did this little exercise because I think that while I've been leaving at least 15% on the subtotal, I've been a little short on the 15% on the total when paying cash. In this case, they'd get more money out of me if I paid with my card!