I forget the exact formula that was told to us by the devs, but I've got some bad news for you.
We graphed it out for different durabilities based on that formula, and the results were the same regardless of where the item started.
Even at 1 durability under max, the chance of max durability loss still exists.
And, at around 30 durability under max, the chance of max durability loss becomes 100%.
If an item can be powdered, you can combat it that way.
If an item can't be powdered, you are best off letting the item get down to 10% or less of its max (25 /255, for example - some people go down to single digits but that brings in a chance of repair failure), then repairing (you'll lose 1 durability, but you'll get the max use from the durability points)