6 perfect skulls + 1 rare item -> 1 low level rare item
ilvl=int(40*clvl/100)+int(40*ilvl/100)

Correct for the portion that will be applied to the jewels alvl calculation. And with the jewel having a qlvl=1 we would get
alvl=ilvl-qlvl/2
alvl=ilvl-1/2
alvl=ilvl
(rings, amulets and Grand Charms are also qlvl=1 making it easier to determine what affixes can appear on them).

QUOTE

new ilvl = (2/3) * clvl (this isn't actually the formula the game uses, but it approaches this limit)

Correct this is an attractor point for the formula. But you might need to run through the iterations of the formula a few times because the truncations of the calculation might force it to stop 1 point off of this mark. You can typically reach the stable point in 3 to 5 reiterations (cubings) even from the worst case starting point(ilvl=1).

Hmmm... ran the reiterations. Need clvl 88 and it will take 5 coming from ilvl=1 to stabilise at alvl=ilvl=57. With a level 99 character you will be above ilvl=57 in only 3 reiterations coming from an ilvl of 1.