When you try to fix something, it feels pretty damn trolly that its total
durability can go down all the way to its current durability. It would certainly
feel more fair if the amount you can "F it up" was capped. It's difficult to
make informed decisions about when to repair what with this huge variance. The
maximum durability should probably drop between 20-40% each time you use the
skill. Another fairer model would be that the max durability can only drop by
the amount the item was repaired for, so e.g. a 5/10 item can only end up being
6/9 or 7/8 at worst. That's a few suggestions.

Note: When I wrote this I had completely missed that you can read books more than once to upgrade skills. I don't know if that alleviates the trolliness in sometimes being able to keep all the durability, sometimes cause it to crash.

Consider it a convenience to get more life out of the random mediocre items you find around the map. When you have nice stuff you are probably already a user of teleport beacons and can thus access Stone's perfect repairs at any time.

BTW: repair skill 9 = perfect repairs + chance to upgrade max durability.