That is a really nice find, well done morninglord! I had some trouble to comprehend the procedure to calculate the poise damage at first though since I am not that well-versed in Souls mechanics. Just in case there are others like me, please allow me to state how I understood your findings (and fell free to correct me if necessary):
Your character has a stat called poise health (PH) which starts at a value of 100. If that value drops to 0 during a fight you will get interrupted and staggered briefly, which is obviously bad. So if you enter a fight and get hit by an enemies attack, this attack does poise damage (PD) to your poise health (PH). This means that after you take a hit your poise health would - in the simplest case - calculate like this:
PH=100-PD
Now if you are wearing any armor the poise stat on that armor leads to an equivalent percentage of poise damage reduction (PDR), e.g. an armor with 30 poise would give you 30% poise damage reduction. So the total poise damage you take would be 100%-30%=70% or (according to the conversion I will use from now on) 1-0.3=0.7. This means that in a more general case your poise health after an enemy attack would amount to:
PH=100-(1-PDR) * PD
Now you say that your character's poise health value will change (usually drop) during a player's attack. The end value of the poise health during this change is calculated by applying a multiplier to the base value of your poise health. The multiplier is based on the weapon the player uses and the attack he is doing with it. This weapon attack modifier (WAM) modifies the player's poise health like this during an attack:
PH=100 * WAM
In case of the 2h r1 of the greatsword you cite a WAM of 21.1% or 0.211 which leads to a drop of the player character's poise health to 21.1 during an attack.
This means that getting hit the first time in a fight and during the player's attack animation will set your poise health to:
PH=100 * WAM-(1-PDR) * PD
If that goes to zero you get staggered briefly.
Now the next phenomenon appears. After a player gets hit for the first time in a fight, him attacking resets his poise health to 80 after the attack, not to 100, except if you are using a weapon art to attack which sets it back to 100. So after you take the first enemy hit the calculation for your poise health during your attack animation (assuming you attacked yourself before taking a second hit) is actually:
PH=80 * WAM-(1-PDR) * PD
This means if you have enough poise to tank through this without staggering and you manage to attack at least once before you get hit a second time to reset you poise health to 80, you should be able to get hit indefinitely without getting interrupted.
The poise damage reduction you would need for this is, according to the formula above:
PH=80 * WAM-(1-PDR) * PD <=> PH/PD=80 * WAM/PD-(1-PDR) <=> PH/PD=80 * WAM/PD-1+PDR <=>
PDR=PH/PD-80 * WAM/PD+1
Taking into consideration that you would want to stay above PH=0 we can simplify this to:
PDR>0/PD-80 * WAM/PD+1 or PDR>1-80 * WAM/PD
Quick test: In your video your weapon and attack used have an WAM of 21.1 and the fireball the player gets hit by does 30 PD, so we get:
PDR>1-80 * 0.211/30=0.4373
Which would explain why you staggered in your video at first when you had 43.72 poise (or PDR=0.4372). Assuming the other values are correct you were 0.01 points of poise away from being safe
I hope this was a correct elaboration on your findings and that this is at least somewhat helpful to people being as overwhelmed by this mechanic as I was at first.
