MyContributionsCalculator does not take into account any alternative methods of retirement savings such as ISAs or stakeholder pensions. You are responsible for carrying out your own investigations before deciding how much to contribute to your pension account.
You should take independent financial advice if you are not sure what to do, or want specific advice about your personal circumstances. You should also bear in mind that there are many issues to consider when deciding how to save for retirement, and the figures provided by MyContributionsCalculator should only be one aspect of that consideration.