A VERY VERY rough example but it may help you out.
Yearly Expenses:
Insurances - £1200
Fuel - £3500
Van - £3350 (£10k van over 3 years)
Vehicle maintenance - £500
Road Tax - £250
Training - £500 (ACS over 5 years plus other courses)
Phone bill - £1000
Tooling and Clothing - £2000
Day 2 Day Sundries - £500
Administration - £800
Just these total: £13,600 (bare with me as these are just examples)
So you need to make £13,600 before you have even started to look at a profit.
Assuming that you work 46 weeks a year (allowing for holiday and sickness) and Monday to Friday that's 230 days per year to earn this. I allow a 4 day week though as you are never always fully booked day in day out but it averages out so... 184 days to earn this!
That's £74 a day before you even earn a crust!!
Good news is you can offset that on tax though but you still need to pay your bills and make a profit so lets say you have a mortgage and a family of 2.4 kids.
Bills will be say £1000 a month so that's another £12k a year needed so another £65 a day to earn.
Total so far £139 per day just to live and stay afloat!
Then you want to make a profit to expand the company or pay off mortgage quicker or put the kids through uni so lets say £10k profit a year. That's another £54 per day.
Total so far £193 per day.
And don't forget the tax man will want his share too!!!
Please bare in mind that these are very rough, plucked out the air figures, and I'm sure there's plenty I've missed, but should give you some idea of what you need to do to work out what your day rate needs to be to be beneficial to YOU.
Jon