to put it all together,
if you have the correct pulley for the head you are using, you can:
A. set timing with NON-adjustable gun on red mark.
B. set timing with adjustable gun (set to 0)on red mark.
C. set timing with adjustable gun (set to 16)on white mark.
Also, if you are 1 or more teeth off on your belt, the distributor will not turn far enough to bring the marks close enough to set the ignition timing correctly.
don't forget your service jumper while adjusting ignition timing