I understand the charge placement on the N & C-terminus and also the utilization of the R-side groups in the amino acids.
For example, What is the overall charge for a chain Glu-Cys-Arg-Asp with pH 11.0 buffer?
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In theory, you can calculate the charge on every individual ionizable group (i.e. looking up their pKa and calculating charge by henderson hasselbalch) and just add them up to get the total net charge.
In practice you would need to measure it with something like isoelectric focusing, since polypeptides fold and certain groups end up interacting with other groups/being shielded from solvent, meaning you really can't calculate it accurately unless you had structural data and even then it would be extremely difficult.