Real racks arn't designed around natural convection based cooling. Any such assumption is bad. You'd have air trapped inside the case and more heat from any units below, and it will all melt down causing all manner of pain, woe and damnation. Hell is probably designed like this. But I'm employing hyperbole here. They'll just overheat, shut down or throttle down, and hell would seem like a nice place for a vacation.
Real racks, or most systems designed for use in any sort of constrained space used forced convection. Real racks suck in air from one side and out the other. That way, you don't need uninterrupted air flow from the top to the bottom.
So the best placement? Make sure the air intake is as cool as possible. Make sure the air exhaust isn't someone elses air intake.
If you need to design a system purely based on convection in a rack environment, either run away, or get a thermal imaging camera and experiment - that should at the very least show you where the heat is travelling, and how close your servers are to melting down.
That said - there's experiments going on with optimising data centre design for higher temperatures and more efficient cooling. These are big projects, done by governments, chipmakers and companies with large data centres, and is out of the scope of the sort of advice you get here.