You're asking how this could possibly work. The amazing thing is that it actually works pretty well, even for your first estimate of the project. But what makes it really work is that you do it again and again.
When you present your first release plan, explain to the management people how you got the schedule. Then tell them that you do not believe this schedule, and that neither should they. You go on:
“Many things can and will change in the course of this development. Customers will change requirements, some things will turn out to be easier than we thought, and some will be harder. That has happened in every project we have ever done, and it will happen this time.
“The difference with this project is that we will ...