Why A decibel change relates to a power ratio by ratio = 10^(dB/10). For a 1 dB loss the remaining power fraction is 10^(-0.1) = 0.794, so about 79.4 percent gets through and roughly 20.6 percent is lost. This is a handy benchmark alongside the familiar 3 dB = half power (50 percent loss).
Watch out The 10.9 percent figure is what a 0.5 dB loss costs (10^-0.05 = 0.891), and 25.9 percent corresponds to about 1.3 dB, so both come from the right formula with the wrong dB value.
1 dB gone = about 20 percent of the power gone; 3 dB gone = half.