A measurement instrument is provided with indicia extending therealong. The measurement instrument indicia is correlated with an indicator of a reference instrument. The measurement instrument is positionable adjacent the entry location of a portal into a patient when the reference instrument is in the portal. The indicator of the reference instrument is correlated to a target location in the patient, and a location of the indicator along the indicia provides an indication of the optimal length for a retractor to be positioned in the portal.