You can use the protparam tool https://web.expasy.org/protparam/ to calculate the theoretical pI of a protein. For a free amino acid with only the alpha-amino and the alpha-carboxy group and no titrable group in the side chain, the IEP is the mid-point between the pKa values of the two groups. If additional acidic or basic groups are present as side-chain functions, the pI is the average of the pKa's of the two most similar acids. see: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Wade)/25%3A_Amino_Acids_Peptides_and_Proteins/25.03%3A_Isoelectric_Points_and_Electrophoresis

For residues within a protein, with both the alpha-amino and the alpha-carboxy group bound up in a peptide bond, you have at most one titrable group, therefore no defined IEP.

Within a folded protein, the electrostatic environment created by neighbouring amino acids affects the pKa of the titratable groups, and it becomes far more complicated, as the pKa of a group is no longer a constant, but varies with pH as the protonation state of neighbouring groups changes and affects the electrostatic field around the the group you are looking at. Tools like the APBS (Adaptive Poisson-Boltzmann Solver) solves the equations of continuum electrostatics for large biomolecular assemblages. https://www.poissonboltzmann.org . By calculating the net charge of the protein as a function of pH, you can, in principle, use this to determine the IEP.