As a matter of ATP scoring, 2K or 2500 seems fine. As a matter of ranking the greats a la TimNZ's interesting criteria, I'd add a few hundred points every few majors, which would allow the tiers to reveal themselves pretty clearly. Maybe 2K points for majors 1-3, 2500 for 4-7, 3000 for 8-10, 3500 11-13, 4000 14-17, and so on and so forth.
I'd also incorporate certain bonuses, such as (1) multiple titles at a given major, (2) most titles at a given major, (3) career grand and golden slams, (4) 3 major seasons, (5) an Olympic gold (which I guarantee will be the 5th most prestigious title in tennis within a generation, if it isn't already - the players already take it very, very seriously since Beijing, and it's only going to grow in prestige in my opinion, and (6) small bonuses for leaders in titles at the YEC and individual masters events.
Not sure what the numbers for each should be, but adding those might create a richer picture. Just throwin' it out there.