Here, it is assumed that the crack is effectively longer than the value measured on the side of the specimen. If one assumes that it is longer by an amount Δ, then the new crack length is (a + Δ).
The term Δ can be determined by plotting the cube root of the compliance, C^1/3 vs. crack length ‘a' as given:

Generally, this approach does not give accurate values since it assumes that the beam is perfectly built in at the tip of the crack.