koko,
Not sure what you mean by "base line". The way it's done is to do kind of a walk back. Start at 5 yards. Note how far below the bull the arrow point is at full draw when you hit the bull. As you move back in 5 yard increments, the arrow point will continue to get lower (farther below the bull) for a while then start to move back up until you reach your point on distance.
The Point on distance is mostly determined by your shooting style. If you shoot three under,your point on will be closer -- maybe 25 or 30 yards. If you shoot split finger it may be 50 or 55 yards. If you shoot with your anchor under your chin it may be much farther.
Knowing your "gaps" at each distance can aid you in shooting. For example, my point on distance is about 55 yards. My gap at 10 yards is 1.5 feet below the bull; at 20 yards it is 2.5 feet; at 30 yards its 3.25 feet; at 40 yards its moving back up to 2.5 feet (the same as at 20 yards); and finally I hold point on at 55 yards.
I just list my measurements to give an example. It will be a little different for each shooter and the ONLY way to get the "gaps" is to shoot and measure them.