You mean add up a player's percentage ratings for each game?
It's not exactly adding up the percentage ratings. To get the percentage, you divide by the total number of points. What I'm suggesting is dividing by the total number of charts. So if there are say 5 charts with 5 records on each of them, and I have 5 points, then there's a max of 25 points. So my percentage would be 80%, but the number added to the overall score would be (25 - 5) / 5 = 4.
For a better example, let's compare the top people on SPA and SA2(B):
On SPA, Rolken is #1 with 38 out of a possible 163 points. There are 51 charts. This would make Rolken's overall score addition (163 - 38) / 51 = 2.45. Now, this may seem rather low, but there's only an average of 3.20 submission per chart, whereas in my first example there are 5 submissions per chart (wow, cyberscore much eh?).
In SA2(B), SuperSonic101 is #1 with 928 out of a possible 11874 points. There are
a shitload of 286 charts: 131 from times, 7 from races, 22 for bosses, yay we ignore freestyle :D, 64 for rings, 62 for score. This makes SS101's overall score contribution (11874 - 928) / 286 = 38.27. Now, this is because there are 41.52 records per chart.
Actually, the maximum number of points you can get is (number of records - 0) / (number of charts) = average number of records per chart. So the higher the average number of records for the game is, the better your score can be. And your score is based on the number of points you have, which is based on how good your records are.
Now, just to compare: the maximum number of points you can have to get the same overall score in SA2(B) as Rolken has from SPA is 11173, which is 5.9%. So it's up to Rolken to see if it's fair to have that much of a discrepency between two games, one of which is vastly unused and the other heavily used.
Edit: It can easily be calculated by: percentage * max points / number of charts, which makes it a lot easier/cheaper to calculate.