I'm looking for the hobby jogger definition of PR-eligible marathon, not the world record definition. For example, I know Boston isn't eligible for world record, but I think if someone told you they ran their PR at Boston (with 460ft drop) you wouldn't automatically dismiss it in your head. The same maybe can't be said for St. George's marathon...with 2559 ft of elevation drop.
I think Boston is my upper limit, so marathons with more than 500ft downhill seem excessive. What's your acceptable threshold?