FWIW, you'd need some profile spec to constrain the problem of what eyeball distance you're looking from. One approach might be to define profiles for desktop,

FWIW, you'd need some profile spec to constrain the problem of what eyeball distance you're looking from. One approach might be to define profiles for desktop, mobile, and projector distances. Is it meant to be accurate? No. But it can stop people from trying to munge angular measures with distance measures.