90% is much to high IMHO. When I started using Nethserver in German there were 60-70% translated but it felt likt that 90%. Even now, when the state of the German translation is just 93% but is quite difficult to find any untranslated strings in the UI.
I would never have startet using Nethserver in German and never starting translating and correcting the rest if German would have been no option to select and try. Therefore I repeat my suggestion to have a relative low limit to encourage other people to use it and translating the rest.
50% of all strings should be a good starting point, because it will cover 70-80% of the common dialogs already.