Defense and ar help in PvP

[Concillian]

#1 When does defense start to give diminishing returns?
  for example,
      If with 25k defense, your chance of getting hit is 35%
      and if with 30k defense your chance of getting hit is 37%
      then the extra 5k defense is pointless i.e. "It's giving you diminishing returns."
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That's not diminishing returns... that's negative return.
There is no case where you will add defense and your chance of getting hit increases... adding defense will always make you more difficult to hit (until you reach the cap)

unfortunately there is no way to figure where you are optimized without knowing your opponent's AR. The function has very good returns until you get over the opponent's AR, then things start going downhill.

Also be careful not to underestimate the chance to hit when it's already very low, as the actual amount of damage taken may be significantly different. For example, compare the case you give.

If you have 25k defense and your chance of getting hit is 35%, and your opponent is the same level as you are, then his AR is 13461. So for 30k defense, he has a 30.9% chance to hit you.

So compare 35% to 30.9% and it looks quite small, only 4% different, but in actuality you will take 11.7% less damage on average (4.1 / 35) some would consider 11% less damage taken pretty significant, others may not. But at least this way you can make the determination based on the correct number, rather than what many do and just look at 4%, which more people would consider to be insignificant.