35 is more appropriate. 40 is out of the question. I don't think he'll want to overschedule himself by playing many 250 events just so he can get to 40 singles titles.
Think about this: His schedule already consists of the 4 GS, 8/9 Masters 1000s (perhaps 7 for one special exemption status that he has a year), at least four 500 events (Memphis, Washington + 2 others), Brisbane, Queen's Club and a few other 250s.
That already is 17 tournaments right there, +/- a few other events. That is quite a hefty schedule already especially for a 28 turning 29 years old. Plus, I don't think he'll want to play too much if the ATP only counts the best # of events per year towards the rankings.