Comment 4 for bug 1938869

If it turns out that this needs to be worked on, Gorka wrote a detailed spec about how the calculations are supposed to be done in cinder. Worth reading because he points out how the Cinder definition of some of the terms are different from they way they're sometimes used in industry:

https://specs.openstack.org/openstack/cinder-specs/specs/queens/provisioning-improvements.html