I thought about this before and the real answer lies with how much you need to earn/ how much you value your time.
If you are a one man/woman sole trader and plod along doing minor works you will definetely be able to charge a less than someone that has 6 vans on the road and has to factor everything else in to the equation.
Also I am not saying small outfits can't make good money, the turnovers will be different but profit may be the same percentage.
I added all my costs which I consider necessary to running a business, i.e. Van, insurances , diesel ,scams, a wage then + profit margin on top of that to give me an hourly figure.
If you ain't walking away with profit you need to rethink the figures.