It would be nice if the process was purely mathematical, but it is not. Often, adjustments need to be made to your rate before presenting to the client. A small percentage can be added, depending on the size and type of project, for unforeseen changes. This is a judgment call for the designer based on the work. Adding a percentage gives you some breathing room to not charge extra for every little new change. As you quote more and more projects, you can look at the hours worked after the fact and determine if you are quoting properly. This will help you decide if adding a percentage is necessary.
Adjustments may also have to be made for the type of work you are doing. For example, logo designs are highly valued, and may be worth more than just the hours needed to complete the work. The number of prints to be made may also affect your price. Finally, the use of the work is very important. An illustration that will be used in a monthly newsletter is worth more to a client more than one being used just once. These terms of use, and their costs, should be discussed with a lawyer and included in your contracts.
Finally, the client may have told you their budget for the project (it is often a good idea to ask). You should still calculate your rate, and then determine if you can complete the job within their budget, or close to it. If you are way over their budget, you may end of losing the job unless you are willing to lower the price to land the job, which can be done either before you meet with the client or during negotiation.
