The golden ratio (Φ) is a catalyst for the development and creation of numerous artefacts in art, music, architecture, and design, manifesting human cognizance of proportion and aesthetic beauty. Over a cultural history dating back thousands of years, scholars have been fascinated by its numerical occurrences in nature, leading to theories of constructional usage in creative practice. Documented testaments are scarce, hence the practical application of Φ usage in major pieces of art is often treated as a myth. The research analysed selected sung-through commercial musicals in search of significant plot and musical elements at Φ points calculated through time duration and bar count methods. The findings were subsequently disclosed, analysed, and applied to the process of creating an original sung through musical The Green Door (2015) by Jane Robertson and Stephen Langston. Documentation of the development process through analysis techniques and musical composition demonstrated a devised time-based numerical system (The Marley System) that could be utilised as a technique for the design of future musical theatre compositional form and is evidenced in sung-through commercial musicals of the West End and Broadway.