Take a long metal straight edge ( I use a steel yard stick) place it between
the two pullies. take a second straight edge and measure down (the distance
recomended)
 in the middle of the two pullies   10 to 15mm  and tighten the alt down.
On my ruler i have measured  this out and i have attached the second ruler at
he disired depth so all i have to do  is loosen /place the ruler,this puts
pressur down for the deflection and tighten down the alt.  Fast and quick it
is right or it is not .