In any salary negotiation, it is essential to know what your absolute bottom line is: what your monthly financial need is. Find out how much you spend per month and use this figure as your basic baseline to work out your ideal salary.
For example, suppose you spend £1,500 a month, and your previous salary was £1,750 a month after taxes and other deductions. However, in changing your job, you would like to increase your salary. A good basic increase to aim for is 20%, which would mean that you should aim for a salary of £2,100 per month (after taxes). A perfect job offer would perhaps include a 40% increase on your old salary, giving you a post-tax monthly wage of £2,450. Your goal in negotiating should be between your salary aim of £2,100 and your perfect salary of £2,450 per month. Try to avoid settling for anything less than your last salary (in this case £1,750); bearing in mind your basic needs of £1,500 a month, a figure that will increase with inflation.
The following table shows a summary of this example; do bear in mind that these figures are based on net (ie take-home) pay: