Sorry if this is already old news. Haven't been keeping up with the thread, and did my own tests on Toughness for a potential future build. No idea how this would work in NG+, but here are my findings for NG.
This is still early, but it was bothering me not really knowing how much of an impact Toughness really had, and at what point it allows you to "poise" through attacks, as well as how much it reduces Ki while blocking. This is just a super preliminary test, and I plan on doing something more exhaustive soon, unless someone else already beats me to it.
I tested this on a level 100+ human enemy prior to finishing the game, which had three attacks with a sword. A lunging thrust, a one-handed and two-handed slash. So if anything, this should be the baseline regarding poise, not the upper limit (at least at this level anyway).
Essentially, exactly 200 Toughness is when you can start attacking without being interrupted. This is also denoted by a blue Toughness number. 199 just didn't do it. It has to be exactly 200 or higher. I couldn't really understand what the purpose of anything lower was then, because at 100 - 199, you have a green Toughness. Well, at exactly 100 is when Ki reduction on block kicks in. It also scales extremely well the higher you go after 100. Below 100 is laughably bad in that regard, and essentially pointless as you'll see below. I couldn't test with 0 Toughness due to adding some innately through Prestige.
Just going to format it by Toughness, Thrust/One-Handed/Two-Handed attack Ki reduction.
Toughness 6 - 66/54/82
Toughness 93 - 61/52/71
Toughness 100 - 52/37/59
Toughness 199 - 41/29/52
Toughness 200 - 29/21/38
Toughness 265 - 26/18/33
Toughness 299 - 24/17/31
Toughness caps at 299, and does not require all Heavy armor to reach. Guard Ki reduction on weapons also stacks with this. Highest I've seen so far is 19.5%, but it might go beyond 20%. Highest I've been able to add to armor was +2.