I don't mind sharing this with you

Each year I used to do something similar, so I could work out how much to put aside.
The figures need adjusting because personal allowances have increased and the tax rates have changed, but the mechanics are still the same. It is taken from a Word document so the alignment has been lost but it is still logical if you look carefully.
Tax 2006 – 2007
£ £
Turnover: (48 wks @ 680) (136/day) 32,640.00
Less estimated expenses: 5,000.00
Net Profit: 27,640.00
Less Personal Allowance: 5,035.00
Taxable Profit: 22,605.00
Computation
Starting Rate 10% on 2,150.00
215.00
Basic rate 22% on 20,455.00 4,500.10
Total tax: 4,715.10
National Insurance
Class 2. (Paid separately)
Class 4 8% of Net profit less “lower profits limit” (£4.745.00)
(As advised by Maidstone tax office)
8% of 22,605.00 1,808.40
Total to pay: 6,523.50
50 Weeks @ 130.00 = 6,500