First, after 30. 1 light years, the distance between Sirius and the earth should be equal to 30.1* 0.25+8.6 = 7.525+8.6 =16.125 light years.

Distance is inversely proportional to brightness: (8.6/16.125) *1.47 = 0.784, so the rating after 30. 1 light years is -0.784.