They are there to help you evaluate the possible consequences of making a particular financial decision. However, they are only a tool - they are not infallible. The actual results of the decisions you make will be materially affected by events, such as inflation, interest rates and investment returns, which by their very nature are ephemeral and variable. They have yet to happen and cannot be forecasted with any degree of accuracy except, possibly, in the very short term.
The calculators are there to give you food for thought and to compare the possible effects of different choices. By all means use them, they are reasonably accurate, but the results are not guaranteed. Only a formal current Lender's quotation will give you the exact amounts available.