"WARNING: balancing a weapon does carry with it the risk of ruining the weapon. If you try to balance a weapon already balanced by you or anyone else, the failure rate is higher then normal."
This is from the help file, and it's not quite true. You can't re-balance a weapon. I think you should be able to, and with a lower chance of success the smaller the skill difference compared to the original balance... Balancing at lower skill than the original carrying a 80% risk of ruining, and needing 60 higher balance skill to reach 20% minimum risk of ruin... or something...