I'm not sure which thread is best for trying to get your head round how booking/routing impacts surcharges? Some info I pulled from https://rtw.oneworld.com/rtw

* NRT-xHKG-JNB-xDOH-LHR-HEL-JFK-NRT is ~$500 in taxes and surcharges if flying AA on the last segment (taxes are ~40% of it)
* Ending JFK-NRT on JL instead of AA adds ~$245
* Dropping HEL (so going LHR-JFK) adds $180 if flying AA or $360 if flying BA
* JNB-xDOH-LHR is $315 less than JNB-LHR (on BA) [possibly only if already flying QR on other segments]
* Adding DOH-CDG-DOH adds $200 [possibly only if already flying QR on other segments] of which ~$80 seems taxes