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add_filter('woocommerce_sale_flash', 'my_custom_sale_flash');
function my_custom_sale_flash($text) {
    global $product;
    $percentage = round( ( ( $product->regular_price - $product->sale_price ) / $product->regular_price ) * 100 );
    return '<span class="onsale">-'.$percentage.'%</span>';
}

add_filter( 'woocommerce_sale_price_html', 'woocommerce_custom_sales_price', 10, 2 );
function woocommerce_custom_sales_price( $price, $product ) {
	$percentage = round( ( ( $product->regular_price - $product->sale_price ) / $product->regular_price ) * 100 );
	return $price . sprintf( __(' Save %s', 'woocommerce' ), $percentage . '%' );
}