And yes, The repair system works such that when you repair an item with Dark Matter, you ALWAYS add 100% condition to it, so repairing an item at 99% will make it wind up with 199% durability, and make it last almost twice as long.
You can’t “waste” durability when repairing manually. It’s a very convenient system.
File that under “things I wish the game actually told me” then… it’s a sizable list, unfortunately. I’d been thinking that 100% durability was the max and manually repairing would waste any excess, so I kept manual repairs for when I was in the field and needed to fix something that’d gotten heavily damaged. And then I got better gear and kept hitting the menders every time I passed one and doing my crafting in front of them and basically stopped carrying dark matter at all.