Chippy
Try approaching it from another angle ....
How much money would you like to earn in a year, comfortably ?
How much 'work' can you get (are you always mega busy, or do you have a few lean days)
If, for example, you can reasonably expect to 'bill' 35 hours a 5-day week, and would like to earn say £35k a year, across maybe 50 weeks, then £35,000 divided by 1750 (hours per year), works out at an hourly rate of £ 20.
If you start work at say 09.30, finish at 1730 and have a half-hour for lunch, then thats £20 x 7.5 = £ 150 per day.
This is the way I worked it out, based on 35 hours a week and a target annual of XXXX. Much of my work is outside, so in the winter months it can be as low as 30 hours a week, but in summer with good weather I start at 0800 and can still be going at 2000, sometimes 6 or 7 days a week if i've got a lot on and jobs queuing up; so can be actively billing 65+ hours a week. Its all swings & roundabouts.
Also, I spend quite a bit on time on leisure during the winter months, so may only average 3 1/2 days a week. It all depends on what you want, how much you want to earn & what else you prefer doing than work