One little technicality that no one seems to notice...
It's not 1/8 or 2/8.

If it were such a precise science, it would be one half of one
half of one half of one half of one half of one half (or 1/64).

I doubt, however, that this is such a precise science that
the soul is ripped cleanly in half with each murder.

Also, Voldemort murdered more than six people and split his soul in more ways than even he could count. Which portion of a shattered portion left him when he made a new horcrux, and which portions of what size remained?

It's not about math anymore. We don't have all the information we need to calculate. All we can know is that it was a very tiny portion of Voldemort's original soul.

I would think that the biggest shards would have been in the first horcruxes he made, which I gather were the diary and the ring.