The reference for calibration of ultrasonic measuring equipment used to detect flaws in metal is the reflection from a set of flat-bottom holes in metal blocks. Early experimental observation as well as an early analysis using a far-field assumption suggested that the response should vary linearly with the area of the hole. Recently, the validity of this linear result has been called into question by new analyses as well as experiment. This paper presents a new analysis of the reflected signal and evaluates the block set which is used for ASTM recommended practice E-127 in light of this analysis.