Great question? I walked down there a couple of weeks ago and it looked like another 10 feet or so for water just to touch the 'lip' of the abyss...3,540? So, I think close to >3,550' would be a minimum usable lake level taking into account trying not to back the average trailer off the edge. That's also assuming they would allow drive axles to be on the soil (say, "slippery and poor traction"). Since the ramp has been high-and-dry, they did move the walkway/dock access to the north and did a lovely job of grooming the exposed ramp at a fairly consistent grade down to the drop-off. That was undoable while it was covered with water, as you know. (I do not think you will see the ramp utilized for anything that cannot be hand-launched for at least a couple of years.) Right now, it seems to be a popular spot for dogs...lol.
Incidentally, I heard rumor the Nation has a lobbying effort/proposal to modify the public ramp so it could be used and is being met with resistance from NPS. Not sure...rumor. If that comes to fruition, probably a $10-12 million-dollar (pure guess) excavation job, minimum.