Not bad suggestion but very bad math. I think this should be way more simple. In case you are using same armor and broken armor piece, you get 50% boost to repairing condition (aka 6=>9, 8=>12, 15=22 etc.)
Now why your math was bad. Right now I can repair combat armor mk3 for 6%, now giving it 15% bonus wouldnt be realy balanced would it. Also mark of the broken combat armor doesnt tell you how broken it is, or how good you can scavenge it for useful parts, thats why my suggestion is about Same armor + Same broken piece rather then broken armor only.