The calculator is incorrect. You are correct.
Tapering off psychiatric drugs is not simply about following what the calculator outputs: namely dropping down by 10% of the previous dose every 3 weeks.
There will be problems, and you'll need to be prepared to overcome your disappointment at holding or going back up to the previous level.
You should always be prepared to reinstate the previous dose, or increase to a previous level. By prepared, this also implies that you will need backup doses, so prepare or order extra.
Put simply, unless you can obtain the doses at a moment's notice, always order or prepare double what you need.
These are the simple instructions, whereas in reality it is unlikely to be simple. Some guides even recommend a whole team, including medical professionals from different disciplines, family members and friends.
This presents a problem. Most psychiatric doctors are in favour of a lifetime's treatment with antipsychotics, and family members are often either authoritarian / in favour of pathology, or guilt-ridden at the state of their relative, and seeking a biological explanation, which a psychiatrist will provide.
Except the explanation won't be real, seeing as there isn't a simple one, and there certainly isn't a chemical treatment that can target a cause which includes relationships and life events.
So failing a support network like that, there is an excellent peer support forum called Surviving Antidepressants
It's important to note that they are not only about antidepressants. They cover any kind of dependence-producing psychiatric drug.
The moderators there will hold your hand, and make sure you've been properly informed to get you started.
Inner Compass Initiative
The Inner Compass Initiative is a new website which includes a withdrawal section. It places much emphasis on preparation before even starting, so please inform yourself before attempting to start a taper.
This is not official medical advice. Unfortunately at the current time there are too few doctors willing to support a taper, but all the same, this website recommends a suitably informed doctor to assist with your taper.
This website and its owner can accept no liability for problems arising from the advice here.
Good luck with your new skills.