The format of the old version [1] is so much better. The independent variable is how long I need to live in my house to break even, not how cheap do I need to find a place to rent.
If you all you care about is being cheaper than a given rent $X, then just look at the How Long Do You Plan to Stay? chart. When the equivalent rent drops below $X, then you’ve “broken even”.
(That’s in terms of buying being cheaper than renting; if you meant making an actual profit on your home, then the equivalent rent has to drop below $0. This typically only happens when the home price growth rate is high.)
But how long you stay is far from the only independent variable. The intent of the new calculator is not just to provide a single, definitive answer — which is essentially impossible given the fundamental uncertainty in predicting the future — but to demonstrate how the variables interact with each other and give you a feel for the space of possibilities. There are many other considerations besides timing.
[1] http://www.nytimes.com/interactive/business/buy-rent-calcula...