I am quite stuck with woocommerce meta_key for discount, am aware of _regular_price and _sale_price This is how i have calculated the discount:

<?php 
            $available_variations = $product->get_available_variations();                               
            $maximumper = 0;
            for ($i = 0; $i < count($available_variations); ++$i) {
                $variation_id=$available_variations[$i]['variation_id'];
                $variable_product1= new WC_Product_Variation( $variation_id );
                $regular_price = $variable_product1 ->regular_price;
                $sales_price = $variable_product1 ->sale_price;
                $percentage= round((( ( $regular_price - $sales_price ) / $regular_price ) * 100),1) ;
                    if ($percentage > $maximumper) {
                        $maximumper = $percentage;
                    }
                }
                echo $price . sprintf( __('%s', 'woocommerce' ), $maximumper . '%' ); ?>