To get the most accurate retirement forecast the key is to be moderate with the annual return rates on your pensions and other investments. Take special care when entering amounts for your pensions pots and contributions.
The goal is to try and make your money last until the end of your retirement (we use 100 years old). There are various strategies for doing this, either you can just withdraw the amount in which your investment grew or you can use the "drawdown" approach. Withdrawing just the interest is a common strategy for those who want to protect their capital (also known as the 4% rule). Have a play with the different options available to see what works best for you.
You will find that the longer the runway to retirement, the more time you have to compound your pensions and investments. Albert Einstein once said "Compound interest is the eighth wonder of the world". Extending your working life by just a year or two can make a considerable difference to your total assets at retirement age.