You would need to crit him 0.50/0.21*1000 = 2381 physical damage (after resil, before armour) in order for the extra armour to reduce the damage by 0.5 which will be rounded up to 1. Need to check the max crits thread - what is the current physical crit record?
That assumes the WoW program rounds with respect to whether the decimal is greater or less than .5. It's also possible that the WoW client just takes the
ceiling of that calculation and used it as it's approximation (in situations rounding to integers). For example, the ceiling of 200.01 is 201, and thus approximated 200.01 as 201, even though 200.01 is closer to 200. Unless one can confirm that it's rounded with respect to .5, or approximated through other means, one cannot be precisely sure.
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For those disagreeing or not understanding what he's trying to say, he's taking into account
round-off error, or whether the difference is noticeable enough to not disappear in outputs
due to round-off errors.
Here's a round-off error example, with a simple program [like WoW, or others] calculating and giving outputs rounded to integers, and [supposedly] rounded with respect to .5:
4000(.021%)=.84 is above .5 and below 1. So it's rounded up, and thus according to the program,
4000(.021%)=1.
However, 1000(.021%)=.21 is less than .5, so according to the program,
1000(.021%)=0.
So, according to the program or rounding scheme, 1000(.021%)=0 implies 1000(.021%)+1000(.021%)+1000(.021%)+1000(.021%)=0+0+0+0=
0, even though the program would say 4000(.021%)=
1.
So, according to the program, 4000=1000+1000+1000+1000, yet
4000(.021%)=1 ≠1000(.021%)+1000(.021%)+1000(.021%)+1000(.021%)=0.