-80% is the change from 255 to 51
Calculation steps:
[( 51 - 255 ) ÷ 255] x 100 = -80%
Formula
Percentage change = [(51 - 255) / 255] x 100
Subtract the new number by the original number
51 - 255 = -204
-204 ÷ 255 = -0.8
-0.8 x 100 = -80%
Example