McFilter

The First Law of Project Management

Features + Quality = End date

Once you work this out, everything else becomes clearer. If you vary one, it affects the others. So if you want to add a new feature, you should expect the end date to slip, and if you don't want the end date to slip, then you need to reduce the features.

[There is probably a more complicated one which involves headcount, but that makes things much more complicated and doesn't work how you'd expect it to - i.e. it isn't (Features + Quality) / Headcount = End date]