An important property of ferrocene nucleobases is the redox potential that can predict their antitumor effect. In addition to being obtained experimentally, using voltammetry, it can also be obtained theoretically by applying quantum chemistry. That is DFT. In order to obtain results of high accuracy and precision, the choice of the DFT method is important. Therefore, the B3LYP method is often used. In this paper, an easier way of theoretical calculation of redox potential was proved by applying the DFT method B3LYP on ferrocenolated purine derivatives with different base sets for iron (SDD and LanL2TZf). Instead of calculating the change in free Gibbs energy for each oxidized and reduced form of the compound in the gaseous and solvated state, its HOMO energy of the reduced form orbit is sufficient. The correlation between HOMO orbital energies of reduced analyte forms and their experimentally obtained redox potentials was based on the previously proven correlation between theoretically calculated HOMO orbital energies of reduced forms of ferrocenolated derivatives, with the base set LanL2TZf for iron, and their experimentally obtained energies. The higher the energy of the HOMO orbital the lower the ionization energy of the compound. Compounds with lower ionization energy are more easily oxidized. A similar correlation was obtained in the paper. The higher the energy of the HOMO orbital, the lower the redox potential of the compound and the easier it oxidizes. The basis set of iron is also important for better linear correlation, which was once again confirmed by the use of LanL2TZf (R2 = 0.9842). Also when measuring redox potentials by cyclic and squarevawe voltammetry, it was confirmed that N7-regioisomers have a higher redox potential compared to the corresponding N9-regioisomers. Voltammetry could be used as a simple and fast method of distinguishing N7- and N9-ferrocenoyl nucleobases.