Although using math's is more complicated than simply guessing a price for quotes and estimates, the extra effort produces a result that is more informed and therefore more likely to be accurate. This more informed result in turn reduces the risk of your projects resulting in lost profitability or even an outright loss—this being where your costs to deliver the project is more than the client is paying you.
The less complicated type of project can in general be estimated as a standard package price—e.g. a 5 page website with bespoke design and conversion to web code to be X hundred pounds, and then Y pounds for each additional page added. This is a reasonable way to approach projects that are low risk in terms of complexity and where you are competing in a price competitive market. Using a client rating based on your knowledge of previous dealings with the client, or of other clients who show similar characteristics to the client in question, allows you to adjust any simple estimates you are considering for the project according to how efficiently you can expect to work with the client based on their characteristics.
More complicated projects can use more detail in estimation to a point that smaller projects may not have the budget to accommodate in such planning. Advanced estimation may still look at identifying a general cost for the entire project, or to invest a significant amount of effort to predict the cost of each phase—a safer approach, that would more than justify any additional time costs.
Self employment and business in general is all about risk management and you will always have to take some type of gamble when taking on new clients. The only difference between a fool who gambles recklessly and a wise decision maker is that the wise person will only take calculated risks based on reliable information. Although you may have little information to start with for calculating the price factors for an individual client, this can be developed fairly quickly to allow you to gradually become more accurate at pricing their projects.