-50.2% is the change from 512 to 255
Calculation steps:
[( 255 - 512 ) ÷ 512] x 100 = -50.2%
Formula
Percentage change = [(255 - 512) / 512] x 100
Subtract the new number by the original number
255 - 512 = -257
-257 ÷ 512 = -0.501953125
-0.501953125 x 100 = -50.2%