on any given level is determined by the enemies you kill with one bomb on the previous level.0 enemies - maximum of 1 letter
1 enemy - maximum of 1 letter
2 enemies - maximum of 2 letters
3 enemies - maximum of 3 letters
and so on
this has helped me so that i am not waiting around for more letters that wont appear. during the lower levels i usually get cristal every 8 or 10 levels.
hope this helps 
