I'm not sure about the 90% figure you quote. I'd say it's a lot less from my experience. But even in that "programming part" I'd say the time to implement core

I'm not sure about the 90% figure you quote. I'd say it's a lot less from my experience. But even in that "programming part" I'd say the time to implement core functionality follows the Pareto principle. You can probably code up 80% of what you need in 20% of the time. The other 80% ends up being QA and bug fix iteration.