In order to monitor unstable but capable processes Castagliola & Vannman have recently suggested a procedure based on an EWMA approach, called EWMA capability chart, for monitoring Vannman's Cp(u,v)-family of capability indices and showed how their proposed approach efficiently monitors capable processes by detecting a decrease or increase of the capability level. The goal of this paper is to investigate the efficiency of this capability chart by computing optimal EWMA parameters yielding the smallest possible ARL's. The method for computing the ARL is explained and a table summarizing optimal values for the EWMA parameters (lambda*, K*) is given for the Cpk chart.