The levitation forces under different field cooling states were measured at 77K by changing the field cooling distance 0Zfc between a YBCO bulk and a permanent magnet. It is found that the relationship between the absolute maximum attractive force (Fmaf) and the corresponding gap distance (Zmaf) to Fmaf can be well described by exponential laws as a function of Zfc, which allow us to predict these values according to Zfc. It is also found that the distance between the Z0fa (gap distance corresponding to the zero force) and Zmaf in the ascending process is a constant value, which is closely related to the constant reduction factor of the axial component of flux density along the axial line of the magnet if Zmaf - Z0fa is a constant value. These results are very interesting for fundamental research and helpful in practical designing and applications.