Kirkgas, I'm pretty much with you on this one. Whilst I can only dream of a day rate of £250 (although I do sometime pick that up on quoted work), I understand the need to charge enough to cover all the circumstances of being self-employed.
You've got 365 days in a year, but let's assume a 40hr week over 5 days, with weekends off, so that's 260 working days.
6 weeks holiday (which is generous but not uncommon in many companies) - leaves 230 days.
8 Bank holidays - 222 days
Sickness (2 and I bet the national average is more than that) - 220 days.
No work available / customer cancels / rain affects play (30) - Leaves our intrepid tradesman with 190 days to earn his living.
Out of his money he has to pay his running costs. That depends on how he is set up, but I'm a cheap Sole Trader so can do it for 10%.
Then he's got to save for his pension. No employer to contribute so he's going to have to put in 10% to stand a chance of retiring.
Tax and NI is 28% (but lets call it 25% to take into account his personal allowance)
So of his 190 days pay, he has to allocate around 35% (need to allow for the percentages to be compounded) to essential items.
Having done the calculations, and knowing what you want / need to take home in a month allows you to fix a day rate. Whether or not a Customer will pay it is another matter.