Rule of thumb according to Mike Sylvester who developed the Serrota fit system and fitted me in Portland: you should be able to hold your back at the angle you ride at without your hands supporting you. This speaks to rudetay's point on core strength. I have a lot of saddle to bar drop on my track bike (5.5") but I have no problems as I've gradually built lower back strength. On road bikes, I run much less saddle to bar drop for more comfort over the long haul.