-50.1% is the change from 511 to 255
Calculation steps:
[( 255 - 511 ) ÷ 511] x 100 = -50.1%
Formula
Percentage change = [(255 - 511) / 511] x 100
Subtract the new number by the original number
255 - 511 = -256
-256 ÷ 511 = -0.5009784735812133
-0.5009784735812133 x 100 = -50.1%