module_load_include('inc', 'calc', 'form.inc') you are including the form.inc.inc file present in the directory where the calc.module file is present.
If you want to include the form.inc file in that directory, you need to use
module_load_include('inc', 'calc', 'form'), as Jimajamma said.
The first argument of module_load_include() is
$type, the second is
$module, and the third is
$name. They are concatenated together using the following code.
$file = './' . drupal_get_path('module', $module) . "/$name.$type";
When you don't pass
$name to the function, it will use for
$name the same value passed for
You could also include a relative path in
$name, such as
'includes/form'; in that case the loaded file would be
'includes/form.inc', where includes is a directory contained in the directory containing the module whose name is passed as second argument.