how would you compare the difficulty levels

[thebeingwithin]

greetings,
i am setting up a tournament for a group of kids of varying levels of ability. we're playing gh5 and metallica. my plan is to first divide them by difficulty level so it's possible to have 5 winners. we all know that those playing on expert are better but i'm trying to come up with a scheme that would equal things out so we can end with one winner and not just the person who does the best on expert.

my original thought is to add bonus points to higher difficulty levels. but i'm at a loss as to how many.

so my question is:
if you play the same song on two difficulty levels how many percentage points do you loose by moving up? for example, if you score 94% playing two minutes to midnight on hard, what's your score on expert?

thanks