I'd love to see your mathematical/statistical reason to say that.
#1 numbers are not used in any other test AND are irrelevant to any end game WAR.
For #2 to #5, parry stay the same, DEX goes up, so does % parry
For #3 to #4, same thing.
#2 IS the control for #5 and the same goes for #3 and #4, we would just need more DEX value with 456 parry and more DEX value with 486 parry, to have a better idea of where exactly is the threshold.

The perfect test would be a test with a fixed parry (let's say 480) and then something like 190/205/220/235/250/265/280 DEX.
Then another one with fixed DEX (let's say 190) and then something like 460/480/500/520/540 Parry.

Then you establish where thresholds are and try to raise both at ones to see if there's diminishing returns when combining the two, if it's exponential or linear,...

It's unfortunately hardly possible without a test server.