Just to add, there's a standard 'cost of doing business' calculation that's probably well worth doing - I did this for myself when I first became self-employed all those years ago, and my accountants now include it in the 'starter pack' for any sole-traders they get enquiries from. I have it in an Excel spreadsheet if the OP (or anyone else) is interested??
The following is the blurb I wrote to accompany the spreadsheet - sounds more complex than it is:-
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Take all your overheads - these are regular (weekly, monthly, quarterly, whatever) costs that occur over a working year, and will typically include rent & rates on premises/workshop, business insurance, all vehicle costs, all likely annual professional fees (accountancy, legal etc...) all phone costs, all advertising and promotional costs, subscriptions to professional organisations, equipment repairs and renewals, health insurances, any additional computer equipment, office supplies, professional development (courses and extra qualifications), internet - broadband, web site & email - basically anything and everything that relates to the general running of your business and isn't tied to a specific job.
Add to this the typical (gross ) salary you'd like to achieve - be realistic about this, it's important!
Then divide this total by the number of billable days you're likely to work. Again, be realistic; there are not 365 billable days in the year, not even close! For what it's worth, I work on the basis of 200 billable days in the year, but obviously your calcs may be totally different to mine - though interestingly 'four days a week on the tools' over 52 weeks only equates to 208 billable days...
This simple calculation ([total likely overhead+preferred salary] divided by [number of billable days]) will give you the day rate you need to achieve to maintain this level. Now, nobody lives in isolation, so if you end up with a day-rate of £490 when everyone else in your area is charging around £180, then you might want to take a look at those calculations again!
Similarly, you might want to leave the 'salary' out of the equation altogether, and work out what your business (or proposed business) is actually going to cost to run, then figure out if you can afford to do it on what the 'going rate' is in your area.
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Cheers, Pete