In Flight Pellet Drop

I'm building an extension to my indoor range by cutting a hole in the wall and putting some glavanized duct out to a box that will hold the target. The duct work will be at yards 19 - 25 of the 25 yard range. 
I'll be shooting JSB 13.43gr pellets around 785 fps and JSB 18.13gr pellets around 885fps. I plan to have 16" diameter duct. The top of the target (25 bull bench rest target) will be about 4" below the top of the duct. I don't think the pellet will drop more than that, (4") but just want to check. Anyone with one of those things that calculates the flight of the pellet able to weigh in? Much appreciated. 
 
Gerry, I'm going to use 16" galvanized sheet metal duct work and then paint it. Should hold up to the elements well. I think I'll just put the light in the box holding the target. Not need to light up the tube, just the target. 

I'll still need to walk outside to put the target in the box, yet this set up will cost me $300-$400, while adding a small extension onto the house would be probably around 5-6K. For that difference, I can stand walking outside in the winter!!