Web Services has been more recently emerged as the technology of choice to realize Service-Oriented Computing (SOC), a significant computing paradigm. The accomplishment of customer's satisfaction and trust is thought-provoking for web service providers. Subsequently, the attainment of non-functional requirements (aka QoS measures) is still a critical research challenge in realizing the Web Service Composition (WSC). The reason behind this research is to design a formal procedure for a service broker to present the worst scenario for users nonfunctional besides functional requirements. We formally address the workflow-based abstract level description of web services coordination through a formal framework of a service broker by composing the functional and non-functional requirements. The syntax of the formal framework is defined and analyzed using π-calculus. However, the semantic analysis of the framework is carried out by considering a case study of the Travel Agent (TA) system. Finally, we present the end-result analysis of QoS measures such as the probability of success, price and average service time by implementing the formal framework.