My understanding of burn down is:

You plan stories based on the expected team velocity. The team velocity is the amount of story points the team assumes they can handle. In the first iteration that is just a guess. The Ideal line shows the assumed completion rate. If you take something closer to the amount of story points achieved in the next iteration and plan that, the plan should be more accurate. You get better over time as the team agrees better on complexities and gains experience.

Now in reality you might add more work to the plan over time. The ideal line should show what you thought you can achieve, and does.

There is another line that shows "expected complete" that shows how much you have to achieve to get there in time. The "expected complete" line, I think, shows what you are asking for in your post. It shows the necessary close rate for the currently open story points versus the time left.

If expected complete is far higher from the ideal line it shows that your planning was not accurate or, more likely, you are asked to do more work in the given time than you can do. If the gap is getting wider you can see it is not going to work. That is the moment when you are supposed to go to your manager and run up the red flag. You are most likely not able to achieve the plan and the project is in trouble.

If you are below the ideal you know you will probably make it without using the weekends and you might be able to pick up stuff from the backlog. You can increase the team velocity for the next iterations.

If you can't finish based on your ideal velocity, you have overestimated what you can do and should lower the team velocity for the next iterations.

If you get good at the team velocity you have a more predictable outlook for later iterations and for the whole project.

So, I think Ideal can probably not be changed and I would consider that a good thing too. 8-)

 Hi there,

 L.S.,

Hello,

Did you get a reply yo this post? I have just had a similar request from my customer! please keep me posted if you know more :)

Many thanks,
Claire

Hi

I am facing same issue. the problem is it takes time to do sprint planning and updating the tasks in RTC. In my case the ideal line is always along x axis. :( as a work around I start my sprint a day later in RTC Sprint plan but that is not right either in this case it always shows my sprint a day shorter. In my view the ideal line should also change as we revise the estimated time.
let me know if some one finds out the solution.

Thanks