From the looks of the formula, the alvl generated would depend on the qlvl of the item. With Giant Threshers
ilvl = 80
qlvl = 89
Since ilvl < qlvl, ilvl = 89
Since ilvl > 99 - int(qlvl/2), alvl = ilvl *2 - 99 = 79
So with the Giant Thresher, your alvl should be 79. In fact, for any rare Giant Thresher that you cube with the 6 pgem + rare formula with any character, you'll get an alvl of 79.
ilvl = 80
qlvl = 89
Since ilvl < qlvl, ilvl = 89
Since ilvl > 99 - int(qlvl/2), alvl = ilvl *2 - 99 = 79
So with the Giant Thresher, your alvl should be 79. In fact, for any rare Giant Thresher that you cube with the 6 pgem + rare formula with any character, you'll get an alvl of 79.