The vibration reduction index Kij is a normalized quantity related to the vibrational power transmission over a structural junction in a building. Its measured or predicted value is used in prediction models in order to determine the contribution of the flanking transmission in the global sound transmission between rooms. The predicted values provided in the standard ISO 12354-1 concern mainly rather simple junctions. This article suggests new engineering approximations for junctions of which the coplanar walls are different. These types of junction are roughly addressed in the standard. The empirical formulas, easy to use, are derived from a large number of numerical simulations and some measurements.