-99.1% is the change from 666 to 6
Calculation steps:
[( 6 - 666 ) ÷ 666] x 100 = -99.1%
Formula
Percentage change = [(6 - 666) / 666] x 100
Subtract the new number by the original number
6 - 666 = -660
-660 ÷ 666 = -0.990990990990991
-0.990990990990991 x 100 = -99.1%