How do you calculate the charge on a polypeptide chain with changing pH?
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.