I tend to repair around 70% as well. It feels like a waste to me of if I repair for less than the max amount, and most clothing repairs 30% max. Gear that I hope to replace later (like cotton socks/gloves) I'll often let go lower especially if I'm low on cloth supplies. Earmuffs are the only thing I'll repair above 80%ish just because they're so fragile. I can never seem to keep them for long.
Sorry for resurrecting an old thread, but TLD is probably my favorite game ever and I'm super excited to have found this community.