Optical coherence tomography angiography (OCT-A) is an ophthalmic imaging technique which has recently been introduced to clinical use. OCT-A provides visualization of the retinal vascularization in three dimensions, without injection of contrast agents. OCT-A could thus replace the current standard of opthalmic imaging, which is 2D only and requires contrast agents. However, quantitative studies remain to be carried out to assess the full potential of OCT-A. In this context, the present work proposes a methodology to perform OCT-A in a more reproducible and precise way. We introduce a procedure to automatically extract the area of interest in avascular regions, which we demonstrate on various avascular areas with a focus on the optic nerve extracted in 2-dimensional images for a selected depth. We then study the repeatability of OCT-A with our segmentation technique when implemented on various clinical devices. For illustration, we apply this segmentation to healthy control group and to patients presenting different stages of glaucoma, a disease of clinical interest. The variability observed between these two cohorts compares favorably to the variability due to instrumental limitations or the segmentation algorithm. Our results thus constitute a significant step toward a more quantitative use of OCT-A in a clinical context.