Hi @afmarchetti,
At the moment, there isn’t an option to add custom validations for calculations. You could add an HTML field with a custom error message and display it based on that as a workaround:

If you are looking specifically for a hook, you could check whether the following would help or not:
forminator_custom_form_submit_errors
add_filter('forminator_custom_form_submit_errors', function( $submit_errors, $form_id, $field_data_array ) {
//Custom validation goes here
return $submit_errors;
}, 10, 3);
I hope this helps in moving forward. Have a nice day ahead.
Best Regards,
Nithin
Hello @afmarchetti ,
We haven’t heard from you for some time now, so it looks like you don’t need our assistance anymore.
Feel free to re-open this ticket if needed.
Kind regards
Kasia