Pendulum sights are advertised as " sight in on the ground from x yards and your dead on 10' -30' up in a tree out to about 30 yards or so" ...

How does one calculate the variables given the geometric nature of the sighting process?

If my pendulum says sight in at 20 yards on level ground, the distance I can shoot accurately out to from a tree is dependent on arrow speed. How do I calculate maximum dead on holding yardage for a given arrow speed?