Smokey, When you get your rammer blanks, take one and stick it in the barrel of your gun. Then, wrap a piece of tape around the blank at the muzzle end of your barrel. Then remove the blank and lay it on top of the gun with the tape in the same position as before. Is it about a half inch from the end of the barrel? If so, you can assume that the gun is unloaded and this is where your breech plug ends.
Then take the rammer and insert it in the RR hole. Is the tape now flush with the end of your barrel? That should give you an idea as to the length of the hole vs the length of your barrel.
If these two lengths are not the same, it means that some obstruction might be in the RR hole. If the gun is missing it's rammer, it's entirely possible that an insect has made a nest or stopped up the hole, or something else is in there.
Your blanks will most likely be longer than you need them. Get your tip installed first and make sure it goes in and out of the hole before you do anything else.
Realize that when you load the gun, the ball and powder will take up a lot of space inside the bore, and your rod will stick out of the bore quite a bit, especially if you use a jag when you load.
Many of us have a range rod that we use while target shooting. It is much stronger than your wooden rod in the likely event that you will dryball and have to pull the ball.
I like my finished rod to be flush with the muzzle when the barrel is empty. If the hole is deeper than the length of the bore, I think people put a mark on the rammer to show an empty barrel.