Put this code snippet at the bottom of your theme's functions.php to modify the flat rate shipping label from (Free) to (will be calculated)

wc-flat-rate-custom-label.php

<?php

add_filter( 'woocommerce_cart_shipping_method_full_label', 'woocommerce_flat_rate_custom_label', 10, 2 );

function woocommerce_flat_rate_custom_label( $label, $method ) {
	if ( 'flat_rate' == $method->id && 0 == $method->cost ) {
		$label = $method->label . ' (will be calculated)';
	}
	return $label;
}