Presently it is believed that the incident power is just the critical power of the nonlocal spatial solitons when the output beam width equals the input the beam width. The power decided by this method is generally not one and only, even is the power of breather. A theory is brought forward that one should find the critical power of the solitons on turning area of the curve of output beam width versus the input beam power. According to this theory, the experimental data obtained in lead glass are fitted, and the exact value of the soliton critical power is obtained.