Published work on computer network reliability frequently uses availability as a performance measure. However, although several ways of defining availability have been proposed, none capture the overall level of service obtained by client hosts in a modern n-tier client/server architecture. We propose such a measure by calculating the fraction of client hosts receiving complete services from the network. We also extend a published, efficient heuristic method for calculating availability to take into account our new proposed measure. The end result is a procedure of polynomial complexity O(nt4), where nt is the total number of components (hosts, links and interconnection equipment) in the network. Numerical results of applying the method to several networks are given.