Well, Ive done some extrapolation from the tendance between 65 and 70 up to 80 and I got similar results (not excatly but near) to the results of StealMySoda. If the people help this could be complete rather accuratly posting:
lvl
gap
xp and sp killing the highest mob you can kill (better giants or Yarkan hunting)
With that information I can try to complete the table, but is need the cooperation of people from all levels from 71 to 80 and with different gaps.
by example.
lvl 72
gap 1
xp 1148558
sp 34388
(yarkan hunting at 72

)
Taking account that value the SP gained is 6405 instead of 6433 that is on the table (which is realy close considering that you will probably die on some job fighting loosing xp but no sp), so this table look very well but only with the help of the people can be finished

.