If only it were that simple.
For me, the two really huge issues that make it impossible to estimate are the following:
1) population distribution is not uniform, so areas can pass capacity of sustainability without the entire world getting to that point;
2) It is extremely difficult to estimate the limits of sustainability because our knowledge (technology) has allowed us to increase production and sustainability as population grows, and one cannot really predict when technologies will do that.
If you go back into the history of this sort of thinking, we have passed the limits that were predicted by earlier researchers (start with Malthus) and yet not hit that brick wall of can't go bigger without a crash.
In spite of that, there is certainly a lot of stress on the system at present populations. Perhaps we have passed the limits of sustainability already but the impacts are not affecting man directly yet. Only affecting the ecosystems for now, with the wall coming at us but with us not recognizing it.
This is not at all an easy question to answer, because no one really knows.