One hopefully final question:

Is there any way to have three stops in continent of origin?

BTW, I think I have revised my plan to the following so any suggestions would be most welcome:

CAI-LHR-SIN-HKG*-BKK-SYD*-MEL*-AYQ*-PER*-SYD-JFK/LGA*-YYZ*-DFW-YVR*-DFW-BOS-SFO-LHR-IST*-LHR*-CAI*

*=stop

total miles = 50940 not including status and cabin bonus

You can see that in NYC I have to change from JFK to LGA as it seems only LGA have direct flights to YYZ (I am assuming this is 'legal' as it is the same city and therefore won't be counted as yet another segment).

Thanks again everyone for your help. Very much appreciated!