Which is the most common site for suck-back porosity?

Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. The entering metal impinges onto the mold surface at this point and creates a higher localized mold temperature in this region, known as a hot spot. A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This, in turn, creates a shrinkage void or suck-back porosity. This type of porosity defect can be eliminated by flaring the point of sprue attachment and reducing the mold-metal temperature differential—that is, lowering the casting temperature by about 30°C. Phillip’s 12th edition page 227