I never said the stats at level 50 are half those at level 100. I know they're not.

But see these formulae

HP = (BaseHP)+(EV/8)+(IV/2)+60

Stat = ( (BaseStat)+(EV/8)+(IV/2)+5 ) * Nature

Since stat calculations are floored, not rounded, it generally requires 8 more EVs to gain one more point in a stat that is neutrally affected by the nature.

Admiteddly there is an exception - if the IV is odd, then four EVs gains the first stat increase.

Thus, for Pokemon that will be used at level 50 (whether they are naturally that level or are auto-levelled to it):

If the IV is EVEN, and the stat is not affected by the nature, the EVs in that stat should be a multiple of 8.

If the IV is ODD, and the stat is not affected by the nature, the EVs in that stat should be 4 more than a multiple of 8.

If the stat is affected by the nature, it's more complicated, and you should probably do the specific calculations for the Base Stat, IV, and EVs in question.