I know this is an old thread but I can't believe how complicated people have made things! Just in case anyone else comes across it and wants to know the simple formula:
The total amount of income, before deductions, is 100%. If tax is taken off at a rate of 20% that means your net income, after tax, will be 80% of your gross income (100%-20%=80%). So the net figure you have is 80% of the gross figure you are trying to find. Simply divide the net figure by 80 to find 1%, then multiply it by 100 to find 100%.
If your net earnings are £800, dividing £800 by 80 gives £10, and multiplying it by 100 gives £1000.