you cant "determine" an MPM.
You need access to one of the paid tools EF/KVS
Aeroplan uses the highly restrictive MPM+5%, here is YYC-JNB via Atlantic and Pacific - use the MPM5 value
[KVS Availability Tool 7.3.6 - Reference: Maximum Permitted Mileage [MPM]: YYC-JNB]
Code:
GI M 5M 10M 15M 20M 25M
MPM AT 11995 12594 13194 13794 14394 14993
MPM PA 15583 16362 17141 17920 18699 19478