Greetings!
Thankyou Maldur!

Well, the answer to your question, I suppose, is when I had a discussion with my players about the feat, *Toughness* In the book, it's obviously quite worthless as a feat. Thus, the need to improve the feat, and make it worthwhile for *anyone* of *any class* to choose to have the feat.
I questioned whether adding just one or two hit points was worthwhile--which it didn't seem to be. The idea of adding hit points of a certain number for each level after you took the feat was discussed, and discarded. It was important that the feat also retained it's "Repeat-Factor"--or the potential for it to be selected more than once.
Finally, I considered the ramifications for making the feat truly useful for any class. I saw that as a prime consideration, and yet, I also didn't want to make something that was unbalancing. Thus, I considered six hit points as a base, which isn't too large, but still useful, and then modified by the character's Constitution bonus. Realistically speaking, most of my players would not take the feat more than once, but taking it once is always a serious consideration. Thus, with say a +2 Con bonus, which is fairly typical for any adventurer, of any class except wizard or something, the extra bonus would be six points. Thus, 6+6=12 extra hit points, for a whole feat! I thought it doesn't exceed the base hit die available to most characters, so that establishes a sort of "outer limit".
It means though that if you have a wizard with no Con Bonus--if you need hit points, an extra six can be nice.
Or, if you have a Rogue or Cleric with a +2 Con Bonus, you would get 6+6=12 bonus hit points, which is pretty cool, I think.
Then, let's say you are a Ranger or a Fighter, with an 18 Con. This would be 6+(3x4=12)=18 hit points. This is for one precious feat that they could use for Spring Attack, or some really wicked ability. Or, they can have an extra hit point boost that actually means something, and is tangible. If considered carefully, an extra 18 hit points is merely an extra Great Sword or Great Axe attack that strikes home. Thus, I concluded that even with a really buffed character, the bonus would potentially "save them" by enduring one more lethal strike. It seemd pretty balanced, and I even querried the group about monsters and creatures having the feat. They all agreed that it was balanced, useful, and now it was a serious consideration for any character to choose. It was with that, I was persuaded that I had designed something that was just right. That is the design process of my adaptation of the Toughness Feat.
Semper Fidelis,
SHARK