Every Pokémon stat since generation 3 comes out of one formula with five inputs: the species base stat, the individual value, effort values, level and nature. For anything other than HP it is
floor((floor((2×base + IV + floor(EV÷4)) × level ÷ 100) + 5) × nature)
There are three rounding-down steps in there, and they are where all the practical advice comes from.
The first floor
Effort values enter as floor(EV ÷ 4). Four effort points become one unit; three become nothing. This is why competitive spreads are always multiples of four and why the familiar cap is 252 rather than 255.
Both 252 and 255 divide down to 63. The extra three points are discarded, every time, in every generation since the third. A spread totalling 508 does exactly as much as one totalling 510.
The second floor, and level 50
The next step multiplies by level and divides by 100, then rounds down again. At level 100 that multiplication is by one, so each unit from the first floor survives: four effort points reliably become one stat point, and the maximum 252 is worth 63 points.
At level 50, which is where most competitive play happens, the multiplier is a half. A unit is worth half a stat point, so it takes two units to guarantee one, and two units is eight effort points.
Step through it with the Pokémon stat calculator on a base 100 stat with a perfect IV and a neutral nature. Zero effort values gives 120. Four gives 121. Eight gives 121 as well. Twelve gives 122, sixteen gives 122, twenty gives 123. The pattern is a gain every eight points, with the first four getting a point for free because of where the halving lands.
The whole 252 is worth 32 points at level 50 against 63 at level 100, which is the single most important difference between the two formats and the reason spreads are not portable between them.
What the other inputs are worth
Individual values run from 0 to 31 and are fixed when the Pokémon is generated. On the same base 100 stat at level 50, the full range from 0 to 31 is worth 15 points, roughly one point for every two IVs. That is real but smaller than most people assume, and it is why hunting a perfect IV matters less than a correct nature or a full effort investment.
Nature is the blunt instrument: ten per cent either way, applied last and after its own rounding. On a stat already in the 150s that is around 15 points for a single decision made once, which beats everything except a maxed effort spread. It never touches HP, which uses a separate formula with no nature term in it at all.
The practical version
Invest in multiples of eight at level 50 and multiples of four at level 100. Stop at 252. Spend the remainder of your 510 somewhere it crosses a boundary rather than leaving it in a stat that will round it away.
And check the breakpoint rather than assuming it: the whole point of two floors is that the last few points you spend are frequently doing nothing, and the formula will tell you which ones. For another progression where the designers put deliberate joins in the curve, see the piece on levelling curves.