It is known that the effect of fluid upon structural vibration is very large with the estimation of vibrational performance on floating structures, therefore fluid-structure coupled analysis method which combine the added mass matrix into FEM has been used since many years ago. Thence pretty accurate eigenvalue of structural vibration can be estimated in many cases of ship structure, but the vibration level can not be estimated so. Then a new method of combination analysis using the experimental modal analysis and BEM was presented by authors. To use this new method, the effect of arrangement of the measurement points upon the solution of the modal-BEM combination analysis must be explained. In this paper, the effect is explained by some concrete parametric calculations on an ellipsoid of revolution. As the result of analysis on the calculations, 21 points distribution at regular intervals is enough for the number of the observation points to apply the method to the vertical bending vibration mode even below 4 node. Then the effect of rotation vector is analyzed, it is explained that the accuracy of the solution without considering rotation vector can not be better on higher vibration mode, if many observation points are set. This is verified by comparing with some experiments in the deep water and the shallow water.