Example: Payout 25% of orders subtotal in points.

add_action( 'woocommerce_order_status_completed', 'mycredpro_woo_purchase_reward' );
function mycredpro_woo_purchase_reward( $order_id ) {

    if ( ! function_exists( 'mycred' ) ) return;

    $order = wc_get_order( $order_id );

    // Do not payout if order was paid using points
    if ( $order->payment_method == 'mycred' ) return;

    // Get total
    $cost = $order->get_subtotal();

    $mycred = mycred();

    // Make sure user only gets points once per order
    if ( $mycred->has_entry( 'shopping_reward', $order_id, $order->user_id ) ) return;

    // Reward example 25% in points.
    $reward = $cost * 0.25;

    // Add reward
    $mycred->add_creds(
        'shopping_reward',
        $order->user_id,
        $reward,
        'Reward for store purchase',
        $order_id,
        array( 'ref_type' => 'post' )
    );

}