I had to implement this same feature recently, unfortunately I didn't find a way to get the percentages just by using the top() API alone, I had to perform two calls, one to nodes() in order to retrieve total capacity and another one to top() to retrieve used capacity. Then it was just a matter of calculating percentage.

A snippet of the working code:

public static void main(String[] args) {
    KubernetesClient kubernetesClient = new DefaultKubernetesClient();
    Map<String, Node> nodeMap = kubernetesClient.nodes().list().getItems()
        .stream()
        .collect(Collectors.toMap(node -> node.getMetadata().getName(), Function.identity()));
    List<NodeUsage> usageList = kubernetesClient.top().nodes().metrics().getItems()
        .stream()
        .map(metric -> new NodeUsage(nodeMap.get(metric.getMetadata().getName()), metric.getUsage()))
        .collect(Collectors.toList());
    System.out.println(usageList);
  }

  private static class NodeUsage {
    private final Node node;
    private final BigDecimal cpuPercentage;
    private final BigDecimal memoryPercentage;

    private NodeUsage(Node node, Map<String, Quantity> used) {
      this.node = node;
      cpuPercentage = calculateUsage(used.get("cpu"), node.getStatus().getAllocatable().get("cpu"));
      memoryPercentage = calculateUsage(used.get("memory"), node.getStatus().getAllocatable().get("memory"));
    }

    private static BigDecimal calculateUsage(Quantity used, Quantity total) {
      return Quantity.getAmountInBytes(used)
          .divide(Quantity.getAmountInBytes(total), 2, RoundingMode.FLOOR)
          .multiply(BigDecimal.valueOf(100));
    }

    public Node getNode() {
      return node;
    }

    public BigDecimal getCpuPercentage() {
      return cpuPercentage;
    }

    public BigDecimal getMemoryPercentage() {
      return memoryPercentage;
    }
  }