Using stellar scintillation to measure isoplanatic angle θ0 is now a widely used method. Based on the theoretical analyses, more accurate calculation formula is derived. With formula derivation and numerical simulation, it shows that the best aperture diameter is around Fresnel length. Equivalent wavelength of polychromatic scintillation and range of turbulence path length are analyzed, and the calculation of proportionality factor C of θ0 and scintillation is discussed. Proportionality factor for Fresnel number in a range from 0.5 to 1.1 can be accurate fitting by cubic polynomials. Proportionality factor should not simply take a fixed value at a wavelength of 500 nm, while it should be calculated by the fitting formula in practical applications.