A .30-30 has a 6" "point blank" range of about 200 yds. What this means is that if you have the sights set to hit 3" high at 100 yds, its trajectory is such that that will be the highest it gets before it starts to drop, crossing your line of sight again at about 170, and be 3" low just a little past 200. They really start dropping fast after that, but if it is sighted in this way, a dead on hold should result in a kill at anything out to 200 without quite as much concern for exact range estimation, holdover or drop.
As Robert said, a dead on 100 yd zero will give too much drop at any range past that if there is a chance that you will be taking any shots at more than 100. At 150 you would be right at 3" low, at 200 about 8 1/2".
bergie