for best results, further is better. 100 would not be a complete waste of time because you'll be able to shoot, but won't do much for your data gathering.
i would consider 200 to be an absolute minimum, but strongly encourage you to shoot 300.
set up a spotting close downrange so you don't have to walk the full 300 yards to check your target, use shoot-n-see targets, etc...
if you walk partway or all the way to the target after each shot, you know your rifle will be in the same condition (barrel/chamber temps) for each shot.
find a way to do it at 300, though. you'll be done w/ that powder in one range session, and the results you get will be as reliable as you are.
good luck!