[ceph-users] Usable space vs. Overhead

I'm having a hard time understanding the EC usable space vs. raw. https://ceph.io/geen-categorie/ceph-erasure-coding-overhead-in-a-nutshell/ indicates "nOSD * k / (k+m) * OSD Size" is how you calculate usable space, but that's not lining up with what i'd expect just from k data chunks + m parity chunks. So, for example, k=4, m=2. you'd expect every 4 byte object written would consume 6 bytes, so 50% overhead. however, the prior formula in a 7 server cluster, using 4+2 encoding, would indicate 66.67% usable capacity vs. raw storage. What am I missing here?