-99.9% is the change from 1000 to 1
Calculation steps:
[( 1 - 1000 ) ÷ 1000] x 100 = -99.9%
Formula
Percentage change = [(1 - 1000) / 1000] x 100
Subtract the new number by the original number
1 - 1000 = -999
-999 ÷ 1000 = -0.999
-0.999 x 100 = -99.9%