Well, I can unfortunately provide some recent first-hand information on this
topic. I died 3 times 2 nights ago at 35 (lousy %^#$ pickup groups with no
tactical sense and hyperactive pullers... grrr...), and each time I lost
perhaps 1/6 of a bubble. EXP resses from lvl 50 clerics restored me to
pretty much where I was before dying - as far as I could see I regained all
my exp, but then the bar was moving very little, so it's hard to say.

Then last night I dinged into 36 and subsequently died twice in SolB (once
when the server crashed and I returned to find myself in sonic bat central -
I never again want to see the words "There is one player in Nagafen's lair"
- but at least my mongo train of 10+ kobolds didn't kill anyone else but me
as I fled). Each time when I died at 36 I lost about 2/3 bubble, and the
exp res only restored about 1/3 bubble.

My theory is this: there is a set amount of experience lost in a death,
which appears to vary (I have noticed this many many times, I die too much)
based on the difficulty con of the monster killing me. In hell levels (30,
35, 40, 45), the bars are effectively twice as long, so the bar moves half
as far when you die, and since my normal exp loss is around 1/3 bubble, when
I die in hell levels I lose about 1/6 bubble. When I die in hell +1 levels
(31 and 36 so far, hoping not to experience dying in the others) I lose
about twice as much exp as normal, or 2/3 bubble. When exp ressed by a 50
cleric, ~90% of that *standard* exp loss is restored - meaning when you die
in hell +1 levels you regain 90% of 1/3 bubble, NOT 90% of 2/3 bubble.

As John has pointed out, the bubbles are very misleading due to rounding
errors in displaying the data, so your results may vary.

Aloro Silvertongue (E'ci)