Round About 100%
Of course we could say that a program existing of many other programs does contain all those and thus should comprise and possess all their possibilities too. But if this saying is associated with total control and perfect survey, than it is more than distracting. The association is definitely wrong.
One could argue that theoretically it should be possible to grasp all possible branches. And with very simple programs running in a highly reliable context this may eventually be practicable. If so, there is nothing to say against making use of them. Yet, trying to reduce the whole computing to such clearly determined programs is absolutely unrealistic. That would mean, on the one hand, closing one’s eyes to unpredictabilities arising time and time again, particularly when and where not expected; and on the other hand, it would mean restricting excessively the ways to use computers.
One great advantage of computers is definitely their ability to deal with situations that are not one hundred percent determined — thus with situations that amount in good approximation to one hundred percent of our real world.
The same is true for the usability of programs, the interactions with the users. The narrower the guidelines are those have to obey, the more the whole becomes unwieldy, nested and therefore intransparent. And it does not even end up providing more safety, but rather increasing the risk of incorrect use.