Engineering companies face the challenge of optimising margins within product platforms while balancing individual product optimisation and maximising platform commonality. Key obstacles include organisational silos, diverse design variables, design space allocation, and varying time perspectives. This paper proposes a value-based modelling methodology that integrates both internal and external variety within the manufacturer. Using an automotive Head-Up Display (HUD) case study, we demonstrate how to effectively utilise platform margins to maximise technological variety and minimise internal variety, thereby enhancing long-term system value. This approach helps design teams understand the implications of their decisions, optimise platform margins to meet evolving technological demands, reduce costs, and maximise value. Our findings advance the understanding of margin optimisation in product platforms and support informed decision-making for engineering companies facing conflicting objectives.