Ticketed point deductions are not cumulative.

Your BOS-MIA-CHI-LON is therefore eligible for the MIA ticketed point deduction only, so you start with a 6400 miles trip (note: IATA ticketed point mileage -- based on ticketed cities--is slightly different from AAdvantage airport mileage), you deduct 1500, and end up with 4900, which is exactly a 25M over the MPM of 3920.

The return is 4808 TPM, which with a 1000 deduction becomes 3808, therefore still inside of the MPM of 3920.