how are the percentage numbers 23.6; 38.2; 50 ... calculated?

First, it is necessary to determine the range of the wave. If we consider 111,959 as the peak, then this pullback process with the most recent low around 90,000 translates the retracement level of BTC as follows.
Length of the wave: 111,959-90,000=21,959
-23.6%: 111,959-(21,959 x 0.236)= 106,776 shallow correction level
-38.2%: =103,571. Strong support level to monitor, usually seen when a long-term upward trend corrects.
-50%: =100,980. Average level
-61.8%: =98,388
-78.6%: =94,700 very deep retracement level, dropping below this may signal a major correction.