You can do a couple things. 1 is to just buy a cheap 30-50 dollar laser bore sighter and sight it in to get it on paper (these can be done at home within 25 yards). 2 is to field trip the entire gun and remove the bolt, look down the barrel and place the barrel on the bullseye. Then without touching the upper at all, adjust the sights to get it horizontally correct and lined with the bullseye. Vertical alignment changes with distance and I would not mess with the front sight post yet. Typically AR's are sighted for 300 meters which is fairly close to 300 yards. Adjust the front sight post accordingly to what type of shooting you do most. I normally vertically adjust while shooting the firearm. If you plan on getting a red-dot, scope or EOTech design, then you can mark the ticks for different ranges. I have marked my EOTech for 100, 200, and 300 yards.