Actually, when I think about it GeneralHPLostRatioinBattle > 0.15 is probably the best option.

For autocalc that means generals which lost more then 1hp.

7hp*0.15=1.05
9hp*0.15=1.35
11hp*0.15=1.65
13hp*0.15=1.95

That should leave enough room for autocalc to award scarred not to often.

For manual battles that just means that general which have 1hp left won't get checked for scarred trait (no big deal).