Suzizila
I would suggest that you do not make any holes in the back of the rosette and leave that to the friend with the bustle. You see the holes will determine how the rosette sits in the center of the bustle and need to correspond to the holes that are in the backboard the bustle is strung on. The rosette needs to placed on the backboard and marked through the holes to insure it lines up right. I would leave it to the lazy bugger that is having you do his beadwork.

One less thing for you to worry about ;)