For me it is simple...an employer will pay you an hourly rate when you start a new job. Over time you have the opportunity to review your skill set and abilities. Based upon those discussions you negotiate a new hourly rate. Why not use this same philosiphy with your own business. Every year since 2004 when I registered my business my hourly rate has increased it keeps a formula such as Peter has outlined above easy to continue to work with week in week out, year on year. In an XL spreadsheet you have just 2 variables with every piece you make.
Les