The EVs required to contribute +1 real stat point is 400 ÷ level, subject to rounding. So, using your examples: 400 ÷ 50 = 8 and 400 ÷ 100 = 4 as expected. This is easily inferred from the stat formula.
I made a graph of it below. Out of interest, I also added labels, each showing the required EVs to get +1 stat points, plus the maximum amount of real stat points that can possibly be contributed by EVs at that level, equal to ceil(63 * level / 100).
Note that the actual breakpoint for stat gains is always a multiple of 4, due to the stat formula. So at level 30 where the requirement is apparently 13.33 EVs, it will actually be increments of 12 or 16 that do the trick. This is why everybody's EV spreads only use multiples of 4.
Also, there won't always be a way to get the maximum gain in stat points shown above, even if you dump all 252 EVs. This is because the stat formula rounds down, so you can get a scenario like this...
(((2 * 85 + 31 + (0/4)) * 1) / 100) + 5 = 7.01 → 7
(((2 * 85 + 31 + (252/4)) * 1) / 100) + 5 = 7.64 → 7
...where it doesn't matter that you put all 252 EVs, because floor(7.01) = floor(7.64) = 7. By the same token:
(((2 * 80 + 31 + (0/4)) * 1) / 100) + 5 = 6.91 → 6
(((2 * 80 + 31 + (36/4)) * 1) / 100) + 5 = 7.00 → 7
I just got +1 stat point by investing only 36 EVs on a level 1 Pokemon, where it's "supposed" to cost 400 EVs. Just how the breakpoints work.
You never have to worry about rounding at level 100, where it's always 4 EVs leads to +1 point, and you can always get your +63 by investing 252 EVs.
