If... 1/2 = semi... Then what's this?

The usual way to express it would be to say '75% official'. Although 'semi-official' technically means '50% official', it is used in a loose sense and nobody would understand it to mean that anything like half of the statement (or whatever) was official. The suffixes 'hemi-', 'semi-' and 'demi-' are all used to mean 'half of something'. The words all come, originally, from the same root but we have 'hemi-' from Greek and 'semi-' and 'demi-' (originally) from Latin. 'Hemi-' tends to be more often used to describe the visible face of something or 'halfness' in the shape of something (as in 'hemisphere'); 'semi-' is often used to express the transition between two states (as in the state between 'official' and 'not official') and 'demi-' usually means 'partly' or 'rather less than completely'. Much of the difference, however, is down to usage and the origin of the word with which it is combined. In musical terminology, the length of notes is described in terms of division by powers of two. A quaver (called an 'eighth note' in American terminology) can be subdivided into two semiquavers, which can be divided into two demisemiquavers and further into four hemisemidemiquavers! One of the best short explanations I could find is here: http://www.english-for-students.com/hemi.html

You can't really measure how official something is. So you can't be sure if what you want to say is really 3/4ths official. It might be anything between not official to semi-official. I think anything that has the tendency to be "somewhat" official can be called semi-official.