Andy, as John correctly said, if you don't charge the correct rate you will nfeel cheated. When i first started in 2002 i cleaned a six seater suite for £45. The guy used to come home in his greasy overalls and the suite, which was velour was caked in grease, especially the arms. It took me 5 hours as i was drying it as well, but it taught me a valuable lesson.
If you want a set amount for a job, hold out and if necessary walk away from it if they do not want to pay. If on the other hand you need the money coming in, take the job. I went through all the same. The more work you get, the slicker you become and you can build a good reputation and progress to the next level.
Selling yourself is an art form and people will judge you on appearance. I always try and go to price every job in person rather than by phone, but some people just want a quick estimate over the phone. With these never give an accurate price, because the chances are when you get there, it is worst than they are letting on, and they think that because you have turned up on the day, you will not turn the job down.
Basically explain that you carry a full professional range of chemicals that can in many cases rectify most problems, but you can never guarantee all stain removal. It took me 3 years before moving from portable equipment to a newish van and new tm if this is what you want in the future, and because we took the time to post tens of thousands of leaflets come rain or shine, rather than just using directories and expecting the work to come looking for you.
Good luck in your new venture.
Dave