The congestion control strategy plays an essential role in the high-speed datacenter network. It aims to deliver low latency, high throughput network service. RTT-based congestion control leverages advanced NIC hardware to identify accumulated queuing delay of the end-to-end path. Sender adjusts the sending rate or congestion window if the delay exceeds a predetermined value, i.e., target delay. Therefore, setting the target delay is the key for RTT-based congestion control strategies. We provide a comprehensive study of the impact of target delay on recent RTT-based congestion control strategies, and demonstrate that a fixed inappropriate target value can lead to low bandwidth utilization or high latency. We then propose a practical queuing target updating approach to solve this problem. The proposed method maintains a shared near-optimal queuing target at the receiving host. We leverage the widely supported ECN flag to estimate the empty state of switch queue instead of indicating congestion, which requires no complicated threshold configuration. We have integrated the dynamic queuing target updating approach into the state-of-the-art RTT-based congestion strategy, SWIFT, and named the design RET. Test-bed experiments and simulations in the large-scale network with synthesized traffic of real workloads show that RET can achieve up to 1.5x and 3.6x lower tail latency than SWIFT and DCQCN, respectively. This paper provides a deep understanding on tuning target delay for RTT-based congestion control algorithms in datacenter networks.