Actually, the devs have stated the "sweet spot" is between 60% and 80% - and even have George Hesphaestus in New Haven (giver of the Blacksmith skill gain quest)state as much.
That said....
I originally had the same qualms about training with talismans, but others have tested and found that the bonus from talisman appears to have only a little effect on skill gain (one can still gain while the talisman puts you over your normal gain range, just perhaps a bit more slowly).
Personally, though, I recommend only using talismans the give exceptional bonuses only during training, and then only if you're training via BOD filling (to make it easier to fill exceptional BODs).