At the time Hari tried to apply it at the small scale, it was impossible. I believe by the time Gaia takes over the science could be applied to the single organism but that also feels like cheating.
The whole point of prelude and forward was that his attempts to start on the smaller scale by going backwards in time was a fools errand and impossible but starting with the relatively small scale of just Trantor was doable as it had sufficient mass to apply the generalized mathematics towards. The variables on the smaller scales were too numerous but as you abstract towards larger bodies, it reduces the complexity to the point it becomes calculable.