The best way to understand the Maestro’s chances to regain the top spot is referring to the Year to Date Rankings
Roger is just 490pts below Novak (9010pts Nole, 8520pts Roger).So for Roger is about bridging that gap. The good news for Federer’s fans is that the Maestro is back to his very best tennis so it wouldn’t be any strange if he has a great tournament in Paris. On the other side Nole is not playing like the “Serbinator” we all know.
Focusing on Paris, Roger can become world no.1 in the Year to Date rankings if he scores +490pts than Djokovic. In order for that to happen Roger needs to win the tournament (1000pts) and Nole shouldn’t go beyond the semifinals (360pts). Roger would gain 640 points on Djokovic to lead 150pts on the Serbian.
That would be rather unfortunate for the Novak, but honestly he has also to cope with a very tough draw in Paris as he is expected to play Isner in the 3rd round, Murray/Dimitrov in the quarters and Nishikori/Ferrer in the semis.
On the other side Roger had a great draw and if he plays like he did in Basel he should get to the final.
Another possible (but truly improbable) scenario is also that Djokovic loses in the 3rd round (90pts) and Roger (600pts) eaches the final in Paris. In this case Roger would gain 510pts and he would lead Nole of 90pts.
After Paris we would have the World Tour Finals and also the Davis Cup that could give 150 points to Federer is he wins both singles.