Back of the envelope (4800-3500) x 1.61 = 2100.
So if we get the 50% S&P retracement to 4150, Primary wave 3 could drop us all the way to 2050 or so.
Fibonacci left the building in 2010 when the Fed blew it away and made a mockery of it. In a potentially Fed-free environment, we might see it regain some stature.
Could apply to a continued rise in LT interest rates, as well. At least until the QE machine starts up again, then all bets are off.