The ever growing demand for data rates motivates the exploration of the low terahertz (THz) frequency bands for mobile communications due to the large available bandwidths. To mitigate the reliance on line-of-sight (LOS), the concept of reflective intelligent surfaces (RIS) has emerged as a promising technology. By reshaping the electromagnetic environment, RIS modifies the channel, necessitating new approaches to model key parameters such as the path gain. This paper proposes the bistatic radar equation as a simple yet accurate model to predict the path gain of a link that includes a RIS, even below the Fraunhofer distance. Chanel measurements at 300 GHz, incorporating a passive RIS, support the validity of this approach.