Mike, you're quite correct. I used the term "approximately 75%" to avoid confusion with the octa descriptions versus the 'tenths' system.
The 'tenths' were replaced, apart from 'metrication', mainly due to the difficulty, in some instances, of an individual deciding what is, for example, 6/10ths or 7/10ths, where to be so relatively precise could actually be difficult.
With, for example, '7 Octas', this is obviously more than half, but less than the whole, so although 88% is the specific answer (which would need angles wings and a very large tape measure to achieve!), then 'approximately 75%', or three quarters of the sky covered by cloud, is good enough to provide a visualisation.
There is a slight difference between 'NSC' and 'CAVOK', especially in Europe, where the former may mean very light, small clouds, in a very wide spread pattern, in an otherwise clear sky, and the latter may mean no cloud, no haze, and visibility OK. I have forgotten what the latter looks like!!