KoBoToolbox is building its forms using a widely shared standard called XLSForm. XLSForm makes it possible to create very complex electronic forms in Excel (or any other spreadsheet program).
Editing a form in Excel can be an easy way to quickly make several changes to a form (for example for translating an entire form).
The XLSForm format is wonderfully documented on xlsform.org.
When using XLSForm you can create complex calculations as referenced in this article