Physically Unclonable Functions (PUFs) allow to generate bitstrings for applications such as device identification, authentication, or key management. For real-world deployment, the industry has stringent requirements on reliability. In addition, as it greatly impacts the security of the whole application chain, the randomness produced by the PUF cannot be compromised. These two requirements are captured by the notions of dynamic randomness-to be minimized in order to improve reliability-and static randomness-to be maximized to increase security. In this paper, we illustrate the whole methodology on a delay-PUF called the loop-PUF. To meet the above requirements on dynamic and static randomness, the PUF's behavior should be modeled and validated; such activities are described in the international standard ISO/IEC 20897. Modeling consists in establishing a stochastic model of the PUF, to predict bit error rates due to dynamic noise, and entropies of the static noise. The model is then verified, its parameters estimated, based on measures in representative environmental conditions.