The construction of a J-R curve for a ductile material following Standard ASTM E 1820-15 ( 2015) requires information on the stable crack extension process. According to E1820, the resistance curve may be obtained from a single specimen test, in which the crack size is measured simultaneously with force and displacement by the unloading compliance, potential drop, or normalization procedures. Based upon the Common and Concise Formats, Donoso and Landes developed the "crack growth law" and the "intercept method" as an alternative to obtain crack sizes in a test that shows stable crack extension, but have only force-displacement data and the initial and final crack sizes available. These alternative methods are now supplemented by an improved key-curve construction based on these formats, and are applied to C(T) test data in which the crack extension values are insufficient to produce a valid J-R curve.