The calculations performed by the SA100 mirror those published by HMRC precisely.
There are a few circumstances where the HMRC calculations do not result in the correct tax being calculated
HMRC has published details about the circumstances when this occurs and has stated that tax payers in these circumstances must file their return on paper. As such, you may find that after you click ‘Calculate’ the form alerts you that you fall into one of these categories and that you must file a paper return.
If this happens to you, you should seek advice from a qualified tax accountant able to perform the calculation for you and then file a paper tax return.