You can't rate purely based on notes per second. If you did, groove DJ would be a 23 and Moonearth would only be a 12. You also have to consider how technically difficult the song is. Bloodrush is obviously harder than Xuxa, for instance, despite being slower. Crossovers, foot switches, funky rhythms, mines, gimmicks, tell jumps, jacks, etc all affect the difficulty. Just compare the song to other songs to judge the difficulty. If it's a straightforward file, 2:00 long, compare it to other straightforward, 2:00 songs. If it's a crazy technical turnfest, compare it to other crazy technical turnfests. If it's a 5:00 stamina song, compare it to 5:00 stamina songs.