Add formatter for computed field [#2623664]

Hello

I'm trying to implement a formatter to a decimal defined computed field, using hooks

hook_field_formatter_info
hook_field_formatter_settings_form
hook_field_formatter_settings_summary
hook_field_formatter_view

but can't get the formatter (my_computed_field_formater) appears in the list of formaters for the computed field (neither on views or content type display settings)

I'm guessing I can't figure out what is the copmuted field "type" that should be defined at hook_field_formatter_info, I've tried

function my_module_field_formatter_info(){
  return array(
    'my_computed_field_formatter' => array(
      'label' => t('My label'),
      'field types' => array('number_decimal'),
      'settings'  => array( 
        'prefix' => '$', 
      ),  
    ),  
  );  
}

is this the correct way to do define the computed formatter on hook_field_formatter_info ?