Like, waiting till there's a certain amount of durability left, using certain items, etc?
For jewels I tend to wait until the durability is at 30 before fixing. Going below that and the chance of a repair failure appears to go up.
For blacksmithed items, get a +60 ancient hammer and make 180 skill repair deeds (you will hardly ever fail a repair). For the rest getting below 20ish durability can often result in a failure (maybe 30%?). I am lazy, I normally wait until I get a warning.
For skills with 100 max I repair at 40, I tried at 30 before I will get a failed repair once every few (within 10) repairs which costs you 2xx durability which doesn't justify for me to risk any further as it's just 10 points difference
For 180 blacksmith you can repair at 0 or 1 and I have never failed.
For 120 tailoring I usually repair at 5-10
“Aragorn’s answer here is v good”
For tinkering, I think the sweet spot is in the 30's. I don't recall too many repair fails if you get it before it dips that low. If you wait until is less than 10, there is going to be a high chance of multiple fails and lots of durability loss. I learned that the hard way, unfortunately.
For tailoring, as long as you repair it before less than 10, you're generally okay if using a 120 repair deed.
For blacksmithing, I don't think I've ever failed with the 180 repair deeds no matter what durability.
They need to give us +100 Tinkering tool to repair the jewelry.