Requesting details about Toughness.
According to the Wiki, toughness adds equally to armor. There is no diminishing returns. For direct damage, toughness + armor is in the denominator, i.e. inversely proportional.
Double that value, you’ll take half as much damage. Quadruple that value, you’d take 1/4 of the original damage.
there is no cap but to get the most effective hp otherwise known as EHP from your gear you must strike a balance between Toughness & Vitality.