Semi-transparent perovskite solar cells have significant potential as the top cell in a tandem structure with silicon (Si) or copper indium gallium selenide (CIGS) as the bottom cell. The efficiency of such tandem cells depends on careful light management and optical design of the device. Here, using the transfer matrix method, we report a detailed optical analysis of the absorption distribution, parasitic absorption and reflection losses in various semi-transparent perovskite solar cell structures and their impact on tandem cell efficiencies. The optical properties of some key layers of a semi perovskite cell are measured and modelled to facilitate the optical analysis. It is found that the encapsulation glass and the absorption in the transparent conductive electrode have a huge impact on the tandem efficiency. The tandem efficiency limit is also calculated to present the design of a two-terminal current-matched tandem device required to achieve 30% conversion efficiency. This work provides directions for further improving the power conversion efficiency of perovskite/Si tandem cells.