The digital device you have allows what some may consider a simplified way to calculate the 3 different aspects of setting up your timing. The up down arrows on the gun are there to allow you to zero the timing tab marker in relation to the damper 0 mark when setting/checking for base, mechanical adv and vacuum advance values.
These are the three amigo's of engine timing in your case.
Essentially there is no difference in what the gun display is telling you that you could not figure out with a simple inductive light without a dial back pot other than you don't need a timing tape that reads up to 60 deg BTDC with a dial back or in this case your digital dial back unit.
Before you go to far it is a real good idea to check that your timing tab/marker aligns with the balancer/damper tab at TDC, as bubba stated I also would highly recommend that you do this first and foremost:
Did i mention I love El Camino's?