Dont know about this method, although i appreciate the input by Krosero. Following the results, Federer had his peak level mostly 2008-12, with the Roddick matches at Wim 2003-5 in between. It is somewhat corresponding with my impressions, that Federer played more serve and volley against Roddick and Philippoussis 2003 at Wim.
Tennis level is always relative to opponent, surface and playing style. I would assume, that most players hit more winners on grass, that one player plays more for winners than another, who is defending better and basically tries to play one more ball back, to win; that is is more difficult to hit winners against fast players than against slower ones and so on.