The current income tax threshold is £10,000 pa (£832 per calendar month, £192 per week). Income above this is taxed at 20% (we’ll ignore the higher rates for the purpose of this question). However, the threshold for National Insurance is currently £7,956 pa (£663 per calendar month, £153 per week). Above this amount you pay 12%.
If you are paying income tax on 25 hours work you must be earning in excess of £7.92 per hour. Your minimum of £192 per week (which takes you into income tax) means you are paying NI of at least £39 per week (at 12% this is £4.48pw).
It is a simple matter to construct a spreadsheet to show you how much your income will be affected by these deductions and how it will vary if you change your hours. However you should also realise that if you are eligible for Working Tax Credits or Child Tax Credits these will also be affected by reducing your hours. You can find calculators online to help you with this though Working Tax Credit should not be effected because you have to work 30 hours per week to get that.