The core Toughness feat seems to be a bit too weak, and I have yet to be at a table where anyone has taken it. I think this improves it.
Adventurer Tier: Increase your baseline hit points value by 1 (i.e., a wizard with bss's Toughness has the hit points of a cleric). You cannot take both this feat and ordinary Toughness.
This feat makes the bonus hp increase linearly per level rather than non-linearly per tier, and grants more bonus hp overall. For example, assuming a barbarian with no CON mod (for simplicity):
Toughness would grant 3 hp at level 1, for 14% (3/21) bonus hp, as would bss's Toughness. At level 4, the Tough barbarian would have 7% (3/42) bonus hp, but bss's Tough barbarian would still have 14% (6/42). At level 5, the Tough barbarian has 12% (7/56) bonus hp, and bss's Tough barbarian remains at 14% (8/56) (and will throughout). At level 7, the vanilla Tough barbarian has only an 8% (7/84) bonus, at level 8 the bonus is 12% (14/112), and at level 10 the bonus is back down to 8% (14/168).
This also benefits lower-hp classes more than tanks. (Classes with 8 base hp very briefly get less hp from this feat, even.) Also, I think it's just easier to grok.
You are free to still take the core Toughness feat if you'd like.