This note presents a concurrent estimation technique that broadens the set of queueing systems where concurrent estimation techniques are applicable. Our technique utilizes a proportional relation and concurrently estimates performance measures with respect to various buffer capacity values in a queueing system. Our technique applies to a queueing system that characterizes a router/switch in packet networks, and unlike existing concurrent estimation techniques, it also works with correlated and feedback controlled input streams. Simulation examples show that our technique concurrently estimates performances of the queueing system with superior accuracy and efficiency.