exactly, for 3 degree slope figure 3 timesyour altitude for descent. ie: @4000' TDZE is 800asl=3200'*3= 9.6 nm from threshold set 1/2 your G/S +a zero ie: 160kts 800fpm, 130 kts=650fpm. approximates to be sure, but then again I can only hold +/- 5 kts, degrees, or +/-100' sometimes close enough is good enough, other times use the damn box