-99.1% is the change from 999 to 9
Calculation steps:
[( 9 - 999 ) ÷ 999] x 100 = -99.1%
Formula
Percentage change = [(9 - 999) / 999] x 100
Subtract the new number by the original number
9 - 999 = -990
-990 ÷ 999 = -0.990990990990991
-0.990990990990991 x 100 = -99.1%