I basically weighted myself down (to around 204 with items, 6,000-something with coin), purchased the AA, then calculated the difference. I can't remember exactly which item it was, but it was some sort of Kiln that weighs 25.5 and had 1 in each bag slot (8 total = 204 weight). My testing resulted in something like a 0.94% reduction, but when calculated as 1%, it was spot on for my examples, including weight up to ~6,553.5 with coin.
This may have changed either in SoF (I haven't tested) or Live (Titanium was released before that comment on Allakhazam), but on the Titanium client, is it correct (this can be verified using
#givemoney &
#showstats). It shouldn't make any difference, but I tested this on an Iksar Monk.
I just checked the
Live Changelog on Allakhazam, but wasn't able to find any references to any changes to Packrat between Titanium & Live, but it may be worth testing on the SoF client.