Instead of using 10 points as in Diehard, your constitution would be used instead if you also have Dieharder(or Diehard 2). So if you have a con of 30, you will gain in effect 20 hit points before you die, equating it nearly with the toughness feat.
The natural extension for diehardest is to add your con to the 10 original points. Thus if you had a 30 con, you could go up to 40 pts negative before dying. This would not be quite as good as the toughness feat.
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