The idea of the 85% rule is more a rule of thumb than some kind of rigid metric--I mean, I'm not sure how you would even quantify that. But I would argue that

The idea of the 85% rule is more a rule of thumb than some kind of rigid metric--I mean, I'm not sure how you would even quantify that. But I would argue that "as long as business features are being implemented regularly" is too vague to be useful. You could be regularly implementing business features and still have half of development time building some framework that is unnecessary.