While there is not much discussion on the importance of formally describing and analysing quantitative requirements in the process of software construction; in the paradigm of API-based software systems it could be vital. Quantitative attributes can be thought as attributes determining the Quality of Service – QoS provided by a software component published as a service. In this sense, they play a determinant role in classifying software artifacts according to specific needs stated as requirements. In previous works we presented an efficient, and fully automatic, analysis technique for establishing Service Level Agreements – SLA. Such a proposal relays on describing QoS contracts as convex specifications, and compliance checking is performed by the application of an analysis algorithm based on state of the art techniques used in hybrid system verification. Such a technique succeeds in offering a procedure for determining SLA but fails in the more realistic scenario where, potentially, no service fully satisfies the requirements. In this scenario the running application may still prefer to invoke the service that offers the best chances of successfully executing with values for QoS attributes meeting the requirements satisfactorily.In this work we propose and implement a metric for automatically quantifying partial satisfaction of QoS requirements, leading to a way of ranking services according to such notion of partial compliance.