Basically, as advertised "tripple miles" for F & J, is twice the economy miles, plus your F 0r J miles plus your status bonus.

Therefore:

PEK-DXB in J - Silver Status =

Y class PEK-DXB 3'631
Bonus 3'631
J Class miles 5'447
Gold Tier (50%) 1'815
Total 14'524

That's how I have been working it out anyway, and I have been more or less spot on.