The increasing adoption of natural fibres as composite reinforcement is a promising development in materials science. These fibres have a low carbon footprint and are biodegradable, and they also have remarkable properties such as low density and high specific stiffness and strength. However, the mechanical properties of these composites are influenced by various parameters, which can complicate comparisons due to their diverse internal structures. This study focuses on two key normalised parameters: the Tsai modulus, which represents the trace of the stiffness matrix tensor; and the area of the Omni failure envelope in stress space. Our analysis of published data on unidirectional flax, hemp, jute, and kenaf composites shows that trace-normalised longitudinal Young's modulus can effectively facilitate stiffness comparisons between natural and synthetic fibre composites. A new and innovative way of measuring strength is suggested. This is based on the radius of a circle that matches the area of the Omni stress envelopes. This method is both robust and reliable for quantifying and comparing material strength. Although, extensive mechanical data on natural composites is available, it is difficult to establish design criteria for comparing them. Addressing this gap presents a significant opportunity to unlock the full potential of natural fibres in composite applications, paving the way for a more sustainable future in engineering materials.