Guided wave tomography is a very attractive technique for online nondestructive inspection and structural health monitoring (SHM). RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective tomography algorithm for detecting, locating and imaging defect in plate-like structures. However, it is not complete for evaluating a damage area, because of the insufficient guideline for the shape factor called ${eta}$. The work presented here introduces the calculation of shape factor for the evaluation of a damage area, as well as the variable ${eta}$ parameter technique to correct a damage shape. Experiment is performed using the guided wave pulse-echo method to find the length of damage on a ray path. In the images resulting from the advanced RAPID algorithm, the defect area and shape showed good agreement. The different shapes of the defect require further study.