When I worked at a consulting firm we liked a single rate. Don't paint yourself into a corner and waste your time by having a complicated set of rates especially if you are a one man/woman show. Take what you are considering as your highest rate, double it, (because if you are like 90pct of people I know your are probably under charging) and use that. (I'm only half kidding here)

The overhead to keep your complicated rate structure straight can be difficult for an organization and especially hard for an individual or one man show. More importantly it makes the sales process more complicated. You spend time laying out your complicated rate plan instead of getting them to agree to a proposal and SOW and giving you a nice chunk of money up front based on that estimate. If you are freelancing you absolutely should get a percent of the money up front especially for a new relationship. You can always offer people a discount rate for larger commitments of work if you like - now you are doing them a favor. Set the expectation of availability early for short term emergency work and don't do an hourly rate at all for this if you can help it. Get good at estimating and give them a (high) project based price that reflects your costs for bumping them to the front of the line.