rochelle, point blank ranging is a useful tool but not one to be used exclusively for sighting in a firearm. Especially one as variable as a muzzleloading rifle. With all the different bullet weights and powder loads that are available for muzzleloaders and also powder types of varying power, you need to shoot and practice at all ranges that you intend on taking game at. What you are referring to is known as point blank sight in and some also call it arch ranging. At 13 yards, your bullet is still on the rise portion of the arch and at 100 yards it could be on the same level on the downward portion of the arch. This, again especially with muzzleloading rifles, can not be depended on to be anywhere near accurate. For one reason, if you sight in to be dead on at the 13 yards first, you will more than likely be in the dirt at 100 because of the sight angle. But if you sight in dead on at 100 you may well be dead on at 10-13 yards. Look at a few trajectory charts and you will be more able to wrap your head around that.