How to determine correct difficulty to play at max level?
This depends on what you are trying to accomplish.
If you want to farm for gear, then you should play on either Normal or Torment 1. If you want Torment-specific items, then play on Torment 1, otherwise play on Normal. The farm speed on Normal is so fast compared to other difficulties that you're going to get so many more rolls of the dice that the minor differences in drop rate are easily overcome. You want to play on the easiest difficulty that can still drop the items you want.
If you want to farm for experience, then you should play on the hardest difficulty where you aren't challenged. This will increase over time as your gear and player skill go up, so there's no rule for how to approach this. You will just have to try all the difficulties and see which is the highest one that you can plow through without stopping.
You can't judge anything based on your Damage, Toughness, and Healing, because these equate to entirely different amounts of DPS and survivability depending on class and skill build. There is no rule like what you're asking for except to just try and see what happens.
This website is filled with good information for Hardcore.
In addition to StrixVaria's answer I would add:
It is important to note whether you are playing on Hardcore or Softcore and the Class you are using, the progression numbers will be different based on those.
I concur with Strix in saying the fastest way to gear up your character is doing speed runs on Normal. On Hardcore, you are going to progress into Torment 1 later than you would on softcore.
Important Class Notes for Hardcore:
- Wizards have a built in resurrection from passive skills (that make progression a little more safe).
- Witch Doctors also have a built in resurrection from passive skills.
- Monks also have a built in resurrection from passive skills and have a built in (hidden) 30% damage reduction.
- Barbarians also have a built in (hidden) 30% damage reduction.
- Crusaders have a hidden toughness bonus from Shields. (other classes too if they use shields)
- Crusaders also have a built in resurrection from passive skills
- Demon Hunters have... mobility and inflated toughness.
Personal Experience
I play on Hardcore and have level 70 Crusader, Demon Hunter and Witch Doctor. It would not be safe/wise to give an absolute value (say 10 or 12 million toughness) for what you need in order to be safe in Torment 1 for a few reasons:
The Demon Hunter's toughness has Dodge built into the value from Dexterity, but when you do not dodge you are taking a higher amount of damage (compared to your EHP or toughness than the other classes), so I consider their toughness to be inflated.
The Crusader's toughness does not factor in Shields (which 99% of crusaders probably use). So the crusader's toughness value is actually understated.
That said, my group of friends on Hardcore moved into Hardcore Torment 1 with a minimum of 10 million toughness and between 400-500k damage. At around 12-14 million toughness and 600-800k damage, farming Torment 1 feels pretty safe (even when pulling a double pack). We have 5 people who play together regularly, all with 2-4 level 70's and so far no deaths. We each only have 1 that we comfortably farm Torment 1 with currently though.
I can't speak from experience regarding Softcore progression but maybe someone else can offer similar guidance in a separate answer.