My "system" constructs a very simple abstraction of what a top-tier player is. The ones in my top tier are:
Tilden, Budge, Gonzales, Rosewall, Laver, Borg, Sampras, Federer
I don't believe it is necessary to fool around with complex numbers, nor do we have to oversimplify things and count majors. Instead let's establish three basic "necessities" of a top-tier player.
a) Peak. A top-tier player is expected to have an extended peak of somewhere in the neighbourhood of three consecutive years as the best in the game. All of the listed players have this; study them enough and you'll see that it's not arbitrary. Some have more than three. Please note that having three years in total as #1 is not sufficient - the peak has be extended and thus years have to be consecutive. Of course, some players may put together three years consecutive as the best in the game and have some other isolated years as #1. That's in their favour.
b) Surfaces/Conditions. I look for the following: grass, clay, outdoor hardcourt, indoor (hardcourt/carpet). Top-tier players have to have extended success (dominance) on at least three of the four conditions/surfaces.
c) Longevity. The least important of the three in my opinion. This is about the years that surround the peak. They don't matter as much and frankly are of more use when rating one top-tier player over another.
...
Let's look at Nadal. He has put together one full year as the game's best. He has three additional and very solid years as #2, but they're not peak years. If he continues to dominate this year and then has another great one in 2010 then he probably enters the top tier. Note that Nadal excels on grass, clay, outdoor (slow) hardcourts, but struggles on fast, particularly indoor, hard/carpet surfaces. It is still 3/4, however.