I heard from the ever wise someone...
and a friend who seems really lucky at alchemy that i works better if you leave a day between + attempts and stones...
From my experience I won't be doing more than 3-4 stones in a row!
The other day I did about 11 stones, the 5 were pretty good
but after that 3 Int1's and attrib assimilation also set in with the rest failing...
the 1st assimilation increased a stat...
the next 2 assimilations brought a necklace and a ring % from about 60 - 0% on both items
On Jewelry a high % like 75% is = to about +2 or +3 btw ^^