After a fall, the rise needed to get back to the starting point: a fall of b% needs a rise of b / (100 − b), as a percentage.
The fall on the left, the climb on the right: both lead back to the same starting line, but the climb is always longer than the fall.
No market data: an exact calculation on the fall you set.
The fall is measured from the peak, the climb back from the bottom, which is lower: −50% on 100 leaves 50, and from 50 it takes +100% to get back to 100.
The “Stock market crashes” tool gives, index by index, the depth of each crash and the time it took to get back to the peak.
This is not investment advice. Past performance is no guarantee of future results.