Can't tell you a 380 but 747 is typically around 14-15 tonnes. The reason is contingency build-up usually based on a depressurisation a couple of hours out of LAX and either return to PHTO or continue to KLAX at 14,000' and then 10,000'.
Depending on weight I'd assume a 380 is about 10% more.
Alternates for LAX are usually based on KONT rather than KLAS.