(7225) Federer Minimum
 Nadal Maximum
 Djokovic Maximum (-1000)
 Del Potro Maximum (-1000)
 Murray Maximum
These are the standings right now. Roger, if he plays nothing until RG ends has 7225 points minimum. Every point he earns from now until RG will add to his point total.
Murray is currently at 10480 maximum points, if he were to win Miami, MC, Rome, Madrid and RG.
Djokovic is currently at 8840 maximum points, if he wins MC, Rome, Madrid and RG.
Currently Murray is now the limiter on Roger's weeks at number one, which he has now clinched until MC. Murray cannot pass Federer until MC, assuming he wins here in Miami and in MC, and Federer earns no points in either tournament. I believe that puts Roger at 381 now, only 5 weeks shy.
Here are my figures
Here are the maximums I get after the Queens for these 4 players.
I don't consider Murray playing MM clay tournaments, only Monte-Carlo, I mean it's already so impossible to think that he will win RG, Miami + 3 clay MS 1000 that it's useless to add that he will ask a WC for a MM clay tournament
As for Nadal and Del Potro I only count Monte-Carlo, Barcelona and the Queens, here's what I have :
Djokovic 11080 (wins RG + 3 clay MS1000 + Belgrade and Queens)
Murray 10925 (wins Miami + RG + 3 clay MS1000 + Queens)
Del Potro 10155 (wins RG + 3 clay MS1000 + Barcelona + Queens)
Nadal 10090 (wins Miami + RG + 3 clay MS1000 + Barcelona + Queens)
Anyway, I think the only reasonable threat is Nadal, in a very lesser extent still Djokovic if he comes back from his current mess and totally reigns on clay season but it's hard to believe.
Only Nadal can really do it, I guess.
Please note that Federer would have 7235 points by then if he withdrew everywhere ((I consider that he does play Miami yet),
which means that the number of points Fed would need in case Nadal wins everything is only 2800 points=10090-7235
2800 points being nearly the maximum he can get if Nadal wins everything and Fed takes part in the tournaments he said (reaching the final in Miami, Roma, Madrid and RG and winning Estoril and Halle he would have 3500 points=600+600+250+600+1200+250),
the equation is quite simple : it all depends on Nadal winning everything imo
But if Nadal doesn't win in Miami, the situation will already be better
I hope I'm correct