Add a helper function for getting the default system of measurement: farm_quantity_system_of_measurement().

modules/farm/farm_area/farm_area.module

@@ -373,7 +373,7 @@ function farm_area_format_calculated_area($measurement) {
   }
 
   // Get the system of measurement.
-  $unit_system = variable_get('farm_quantity_unit_system', 'metric');
+  $unit_system = farm_quantity_system_of_measurement();
 
   // Switch through available unit systems and generate a formatted string.
   $conversion = '';

modules/farm/farm_quantity/farm_quantity.module

@@ -86,13 +86,23 @@ function farm_quantity_settings_form($form, &$form_state) {
       'metric' => t('Metric'),
       'us' => t('US/Imperial'),
     ),
-    '#default_value' => variable_get('farm_quantity_unit_system', 'metric'),
+    '#default_value' => farm_quantity_system_of_measurement(),
   );
 
   // Return it as a system settings form.
   return system_settings_form($form);
 }
 
+/**
+ * Helper function for loading the default system of measurement.
+ *
+ * @return string
+ *   Returns either 'metric' or 'us' (defaults to 'metric').
+ */
+function farm_quantity_system_of_measurement() {
+  return variable_get('farm_quantity_unit_system', 'metric');
+}
+
 /**
  * Define information about available quantity measures.
  *