Temperature- and magnetic field-dependent resistivity is a crucial parameter in determining the physical properties of manganites. The first objective of this work was to find out an applicable method of using temperature to predict the resistivity and control the transition of La0.8Sr0.2MnO3 and La0.67Ca0.33MnO3 from the insulator phase to the metal phase in the transition area. Predicated on nonlinear curve fitting, a typical numerical method is used to quantitatively analyze the temperature-dependent resistivity for temperatures both less than and higher than the metal insulator transition temperature (T-p). The simulations agree very well with the observed spectra (resistivity versus temperature). The second objective of this work was to find out the mathematical relationships between the magnetic field and the maximum resistivity rho(max) as well as the metal insulator transition temperature, T-p. On the basis of the calculated results from the nonlinear curve fitting method, the magnetic field-dependent T-p and rho(max) are theoretically explained successfully via three functions. The minimum correlation coefficient between the actual and the calculated data is 0.994. The average relative errors between the measured and the modeled quantities do not exceed 5.86 % in all considered cases. To my knowledge, the effects of the magnetic field on the shift of both the maximum resistivity and the metal-insulator transition temperature of manganites are quantitatively discussed for the first time.