Since the corrosion is internal, the only way you are going to be able to get detailed measurements from the outside is with UT. The SLOFEC method will find the corrosion area, but won't give you detailed profile measurements you will need to run RSTRENG. Use automated UT scanner, or use manual UT, if manual someone will have to draw a grid on the outside of the pipe and search for the deepest area of corrosion in each grid square, somewhat time consuming. Also, if manual UT make sure they have a smaller diameter probe, don't use a typical half inch probe, as it is too large to get enough detail, use a pencil probe or small diameter probe. You have to specify, otherwise they typical NDT "company" or "guy" will not care but give you garbage results. By the way, 19% in depth you are right on the limit of even being able to use RSTRENG as the feature is not deep, based on your parameters you don't even fail B31G, you pass with a safety factor of 1.65, and RSTRENG near 2.