Never price up a job on an hourly rate, and never tell the customer what your hourly rate is.
It's fair enough to charge a 'DAY RATE', ie, you charge £150 per day (or whatever it is you think represents the turnover you want to make)
An hourly rate almost never equates to a day rate, most of us are lucky if we work 5 or 6 solid hours a day, take out tea breaks, dinner breaks, ttravelling between accounts, talking to customers, writing out invoices and so on, and that's just for starters.
If possible, always price per job, if you think it's 4 hours and it takes you all day, well..tough, you screwed up, live and learn.
As you gain experience you will make fewer and fewer mistakes, there will also be times when you might reckon on 4 hours work and get it done in 2 hours, but thats what pricing is all about.
Ian