From your actual skills, we can tell that you gave 120% of you! Thanks for the example you reported, probably it's just a matter of "reading the game" sometimes! And there's a lot of factors involved; the client type, your value, if the client is giving you a lot of work... so in the beginning it's not easy to set a price properly but I guess with experience this will come easyer! Anyway I personally found very helpful to do an estimation based on "work per hour" as you do, to have an idea if you are completely wrong or your on the way!