sharpening adds an agility requirement to a weapon
level 1 +6
level 2 +13
level 3 +21
....
so the formula seems to be
previous level requirement + new level + 5
It could be that this is wrong and only works with the stepped use of sharpening (did it in steps, first lvl 1, then 2, then 3, etc whenever my chars had the required agility)
And your question about a good level of sharpening.
To the point where you can still use the weapon due to the added requirement.
