Howdy
Welcome to the fun world of Black Powder Cartridge loading.
Did you really expect to achieve perfection the first time out?
Next time, first of all bring a really, really big piece of paper. Really, really big. And bring more than five cartridges. A lot more.
Your experience only speaks about the point of aim vs point of impact. You did not fire enough rounds to have any idea of how accurate, or ballistically consistent, your rounds really are.
Next time, set out a really, really big piece of paper. Twice as big as you think you need. And come back to 50 yards, don't try to shoot at 100 yards until you know what is happening at 50 yards. Pin a target near the top, or draw a nice big black circle. Then fire a group. FORGET ABOUT TRYING TO ADJUST THE SIGHTS TO GET YOUR POINT OF AIM AND POINT OF IMPACT THE SAME! You can do that later. Just shoot a group, without varying your sights at all. Don't even look through your spotting scope to see where you are hitting. Just use a good rest, and your best trigger technique, and shoot a group. Maybe three shots, five would be better. After shooting a group, then see how accurate your loads are. Once you fire a couple of groups, and have a good idea of how accurate the loads are, then, and only then, adjust the sights to hit where you aim.
This is the most authoritative book there is on loading the 45-70 with Black Powder. If you want to get serious about shooting 45-70 with Black Powder, buy this book. This book was written specifically about reproducing 19th Century 45-70 performance in the Trapdoor Springfield, but it the information will serve well for any rifle. For what its worth, Wolf lists the muzzle velocity of the 45-70 round with a 405 grain bullet at 1350 fps.
http://4570products.info/Loading-Cartridges-for-Original-45-70-Springfield-1.htm
A couple of things. What you are going to discover if you take the time to do the research is your rifle will eventually tell you exactly what it likes. There is nothing sacred about the figure 70 grains. The old ammo had more powder capacity than modern brass does. The original folded rim, Benet primed, copper cases had the most capacity. Later Balloon Head cases had a bit less. Modern solid head brass has less case capacity than any of the old brass. So if you are cramming 70 grains of powder into a modern case, you may be cramming in too much.
Other variables to consider when working on an accuracy load for 45-70 with Black Powder.
The amount of compression.
Thickness of the wad between powder and bullet.
How are you keeping the fouling soft between shots? Blow tube? Long slow breaths are best with a blow tube. Breathe deep to saturate your breath with moisture, and let it out long and slow into the bore.
You really don't need a chronograph for this. It is a nice touch, but knowing your muzzle velocity is really not going to tell you much. Good shooting technique and good loading techniques will tell you much more than a chronograph will. That's why you start with a really, really big piece of paper.