GravityView: Show the result of a calculation using a Custom Content field

gistfile1.php

<?php 
/** Modify the content returned from the Custom Content field */
add_filter( 'gravityview/fields/custom/content_before', 'my_gv_custom_content_before', 10, 1 );

/** 
 * Replaces the %CALC% placeholder by the result of a calc operation between entries' fields
 * 
 * @param  string $content Custom Content field content
 * @return string 
 */
 
function my_gv_custom_content_before( $content ) {
	// this is the tag used inside the Custom Content Field
	$tag = '%CALC%';
	
	// Checks if this custom content field is the one we will use to perform calculations
	if( false === strpos( $content, $tag ) ) {
		return $content;
	}
	
	// Remove the placeholder from the field content
	$content = str_replace( $tag, '', $content );
	
	global $gravityview_view;
	extract( $gravityview_view->field_data );
	
	if( isset( $entry['15'] ) && isset( $entry['37'] ) ) {
		// sums up field #15 with field #37
		return $entry['15'] +  $entry['37'];
	}

	return $content;
}