Pulsed laser ablation in liquid is a very powerful technique that allows to synthesize colloidal solution of nanoparticles, starting from a target and a solvent, without waste. The performance of this process in terms of ablated mass per time depends on the laser fluence, the energy delivered per unit (or effective) area. Experimentally fluence evaluation is a tedious and indirect process, since the optical path passes through a lens and through an air–liquid interface, which is not easy to observe. In this work, a computational approach that provides the experimental parameters is developed, the laser ray paths inside the experimental apparatus are drawn, and the effective fluence on the target surface is evaluated. It aims to be an instrument to make predictions and help in the experimental setup tuning; moreover, the data obtained fit the theory developed for picosecond laser. The predictions can be extended to various real situations like a rotating fluid with a nonplane air–liquid surface.