The farther you ladder test, the better. I do all of my ladder testing at least 750 yards . Lots of time at 1000. That is where you will REALLY see your verticle dispersion.
Helpfull hint.....color your bullet tips and use white backing paper. I like to use colors that really show up on target. if there is a question on color, simply use a q-tip and a little alcohol and dab the bullet hole. This makes plotting your shots VERY easy....a lot less running back an forth to the target.
shot 1 red
shot 2 blue
3 green
4 yellow
5 black
6 red/blue
7 blue/green
8 red/green
9 black/yellow
10 red/green
I use a 4 x 4 sheet of plywood covered with white freezer paper. i use a small aiming point, and I like my shots to miss the aim point to the side by at least 6 inches...that way all of the impacts are in the white.
Edit...color bullet tips only...use colered felt markers to color the bullets, but only color the tips...just short of the bullet ojive. Do not color the bearing surface.