trailling timing
 

i have an 85 rx7 but when i bought it the guy said the timing may be off a bit,so i checked it and it was,i adjusted the timing with the distributor which also advances the trailling now if i turn the knob on the timing light to 20degrees how do i know if its at 20degrees and how do i adjust the mechanism that are located on the side of the distributor and do i have rev the engine to adjust it,thanx i just want to make sure im doing it right