Integrating sustainability into early product design stage is crucial for companies to navigate complexity effectively. Based on a long-term research collaboration with an aerospace product manufacturer, this paper introduces an improved prototype of the Sustainability Criteria and Product Life-Cycle Data Simulation digital decision-support tool for visualising and comparing the sustainability implications of product design concepts. Unlike existing tools, it allows quantitative comparisons in early design stages and is based on overarching socio-ecological sustainability principles and a backcasting perspective. A sustainability merit score for each product design concept is derived from selected indicators and sustainability criteria. The paper also reports on the results from a focus group evaluation study and discusses the value and challenges faced when developing this type of tool, including accuracy, data availability limitations, and the dilemma of indicator weighting.