Constrained Application Protocol (CoAP) is a popular UDP based data transfer protocol designed for constrained nodes in lossy networks. Congestion control method is essentially required in such environment to ensure proper data transfer. CoAP offers a default congestion control technique based on binary exponential backoff (BEB) where the default method calculates retransmission time out (RTO) irrespective of Round Trip Time (RTT). CoAP simple congestion control/advanced (COCOA) is a standard alternative algorithm. COCOA computes RTO using exponential weighted moving average (EWMA) based on type of RTT; strong RTT or weak RTT. The constant weight values used in COCOA delays the variability of RTO. This delay in converging the RTO can cause QoS problems in many IoT applications. In this paper, we propose a new method called Flexi COCOA to accomplish a flexible weight based RTO computation for strong RTT samples. We show that Flexi COCOA is more network sensitive than COCOA. Specifically, the variable weight focuses on deriving better RTO and utilizing the resources more. Flexi COCOA is implemented and validated against COCOA using the Cooja simulator in Contiki OS. We carried out extensive simulations using different topologies, packet sending rates and packet error rates. Our results show that Flexi COCOA outshines COCOA and can improve QoS of IoT monitoring applications.