Jim Pugh Jr. Recently told the Orlando Business Journal that when he does a real estate deal than he will put in 33% in equity and then borrow the rest. This allows for a lower occupancy and still remain cash flow positive.
This is quite conservative and I think it's a good yardstick to go by. It's hard to argue with the man's success. I would go one step further though. I think you need to put down however much is needed in order to be cash flow positive while having conservative figures.
This will reduce your return on the investment. However, the real estate development world is littered with bankrupt developers. It's almost always due to being over leveraged.