The 50% is a very important retracement level and is actually based on two Fibonacci numbers in the sequence. Fibonacci sequence is a sequence in which each number is the sum of the two preceding ones starting with 0 such as this: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ... So from this sequence, if you divide each number by the subsequent number, this is where you get the ratios that are referred to as "golden ratios" or the fibonacci retracement levels that's used in financial trading. For example, if you divide 0/1, you get 0 which corresponds to 0% retracement, you divide 1/1, you get 1 or 100% retracement, after that if you divide 1/2, you get 50% which corresponds to 50% retracement, and if you divide 2/3, you get another significant retracement level of 66.7%, and the reverse of the 66.7% (1 - 0.667) is the 33.3% retracement level. These are actually the original retracement levels based on the fibonacci sequence. Today, people have added more levels such as 23.8%, 38.2% and etc. because it is somehow observed that the price tends to reverse at these levels which sometimes imo is more of a self-fulfilling prophecy than anything else. https://en.wikipedia.org/wiki/Fibonacci_number
If we go by 2008, SP500 dies from 1550 to 666. The equivalent drop would see SP500 die from 4800 to 2062! Not a far cry from 1700!
%% 50 is real good average also\50dma ; but its also a good front run off 55dma or 55 days/LOL. 50% also is a much advertised sale price. Local grocer[Cash Savers] priced Short + Sweet Strawberries @ $1.00 per pound..............................................................................[Sold/buyin' them \strawberries, not penny stocks]
If we go by Oct. 19, 1987 the Black Monday, SP500 dies at 224.84. Here is a handy fibo retracement level calculator: https://www.omnicalculator.com/finance/fibonacci-retracement All the levels are there.