Those numbers make sense, and bring back some memories. The only
airplane maneuver in which pivotal altitude comes into play, as I
remember it, is eights-on-pylons. And I seem to remember that for a
Cherokee (airplane) at 90kt, the pivotal altitude was 700'. Your
formula gives just about that. The pivotal altitude changes as you go
around a circle, though, because unless there is no wind, your speed
keeps changing.