Riblets, small streamwise-aligned surface protrusions, are a well-established method for turbulent drag reduction. This study revisits the mathematical framework of protrusion height to quantitatively evaluate riblet performance. By decoupling the Navier–Stokes equations into streamwise and cross-flow components, we detailed the derivation of the protrusion height parameter that robustly characterizes drag reduction efficiency. Using boundary element method, we compute the protrusion heights of common riblet shapes and confirm that sharper tips generally yield superior drag reduction. However, optimized riblet configurations can be devised that maintain high protrusion heights while featuring strategically blunted tips—a design compromise that preserves drag reduction performance while enhancing durability against tip erosion. Furthermore, our analysis reveals the formation of coherent streamwise vortex layers above riblet tips, providing new insights into their role in modulating near-wall flow dynamics. These findings advance our fundamental understanding of riblet mechanisms and provide practical guidelines for designing high-performance, durable riblet surfaces.