The mixing of different A-site cations in lead halide perovskites is an effective method to stabilize the phase and improve reproducibility together with a promoted efficiency of power conversion. However, due to the complexity of various competing interactions of solvents in the precursor, the optimization of the composition of the perovskite is largely based on the traditional trial and error method. Herein, we explore the interaction between lead iodide (PbI) and typical A-site cations including Rb, Cs, methylammonium (MA) and formamidinium (FA). Based on the calculated amount of charge transfer from these cations to PbI which acts as an inorganic lead source, we quantitatively predict that the ideal recipe of mixed cations for A-site substitution should be inversely scaled by MA : FA : Cs : Rb = 1.00 : 1.65 : 1.48 : 1.42 (normalized) which should be further adjusted for iodine deficient PbI. Our work reveals a stronger amount of charge donation from FA than from MA which may account for the issue of phase conversion of FAPbI. In addition, we propose the mechanism of the initial growth of perovskites via ionic diffusion driven by a built-in electric field across the adsorbate-PbI interface. Our work sheds light on the atomic scale mechanism of redox and ionic processes governing the formation of low-entropy perovskites from the complex ionic precursor solution.