During my years of coaching, the #1 question that players ask is in regard to the correct racket angle when looping.  Having the correct racket angle for each particular stroke is important.  However, there are many many many factors that need to be calculated within a split second to adjust your racket angle to properly loop the ball.
 
#1 The height of the incoming ball
#2 The depth of the incoming ball
#3 The speed of the incoming ball
#4 The type and amount of spin on the incoming ball