-49.8% is the change from 255 to 128
Calculation steps:
[( 128 - 255 ) ÷ 255] x 100 = -49.8%
Formula
Percentage change = [(128 - 255) / 255] x 100
Subtract the new number by the original number
128 - 255 = -127
-127 ÷ 255 = -0.4980392156862745
-0.4980392156862745 x 100 = -49.8%