High-intensity focused ultrasound (HIFU) thermal therapy exploits concentrated acoustic energy to ablate pathological tissues with millimetric precision deep in the body. Accurate prediction of thermal effects is essential for tuning the treatment shooting parameters—such as source pressure amplitude and sonication time—as well as for maximizing efficacy and preserving surrounding healthy tissue. This study presents a computational model developed in COMSOL Multiphysics to simulate the physics of HIFU thermal phenomena, accounting for acoustic propagation and heat diffusion in biological tissues. The model was validated through experimental tests on ex vivo chicken breast tissue within a robotic ultrasound-guided HIFU (USgHIFU) platform, with lesion dimensions serving as the primary metric for validation. Building upon the validated simulation, we define a polynomial-based model that analytically predicts lesion dimensions based on the input shooting parameters. This approach significantly reduces the COMSOL computational cost and execution time, making it well-suited for integration into a real-time treatment planning workflow for clinical use. A desktop application implementing the inverse formulation of the polynomial model was developed, allowing shooting parameters to be computed from target lesion dimensions through a simple and intuitive interface. By enabling a rapid estimation of lesion size, this solution supports a more standardized strategy for non-invasive oncological therapies.