This article describes a comparative study of the accuracy of proportional ratio estimations on stylized optic nerve head targets by experienced optometrists, student clinicians and beginning optometry students. Within limits, these estimations approximate the perceptual and cognitive tasks involved in the clinical estimation of cup-to-disc (C/D) ratios. There was no significant difference (alpha = 0.05) in the accuracy of estimations relative to clinical experience. The overall frequency of exactly correct paired estimations (i.e., both horizontal and vertical correct) for the 16 stylized targets was 18.6%. In a subanalysis of the data, less than half (45.4%) of the estimations were within +/- .05 of the true "C/D" ratios. Statistically, observers were more accurate for the smallest targets and less accurate for the medium and larger targets. Potential clinical implications of these findings, relative to C/D ratio documentation in glaucoma diagnosis and management, are presented and discussed.