It's just my personal approach, so it won't be to everyone's taste.
As already advised, work out how much everything to do with running the business costs for a year. Then add 15% to it for potential increases, and the little bits you forget. Then add to that what it costs you to live for a year. The final figure is what you must earn. Divide that by ten (ten working months), then divide that figure by 20 (working days per month) and see what that comes to as a 'must have' day rate. Once you have your base day rate adjust it up to a reasonable figure for a chargeable day rate to include a proffit for the company.
I do it that way for two main reasons.
Firstly a day rate can be charged as 1/4 1/2 or full day. Nothing takes less than 1/4 of a working day (2 to 2.5 hours if done on an hourly rate) by the time you include paperwork, collecting bits, restocking the van for bits used that way, travelling etc etc. So by using a day rate it simplifies pricing jobs.
Secondly it allows me to monitor my personal and business costs. So if I find that my 'must have' day rate is ÂŁ500:00 then I need to cut something back/out. The end of day massage and manicure for example.
It's a bit of a ball ache to get the figures together the first time, but after that it's a big help. It also allows you to adjust rates according to the job or work. I know for example that a CU change is day rate plus parts, but if I get a couple of weeks work I know how much I can drop my rate by to get the work without making a loss.
I'm not saying it's the right way, or the best way, but it works for me.