As with any load, you need to work up to what works and YOU concider safe.
I started with 50gr (a real pip squeak) and worked up.
Like you I asked a lot of questions and made my own decisions.
Here is a link to a guy who is well respected.
http://members.aye.net/~bspen/SmoothboreLoads.htmlGive it a read and burn some powder _ That is the fun part.
80gr of powder seems pretty standard - So in the begining you might want to use it as a max load and play with different combinations of ball,wad and lube and see what kind of results you come up with.
One thing I figured was a shot charge weighs more than a ball - So if 80 was safe for shot, it should be no problem with a ball.
I know for sure my 55gr load will kill a whitetail stone dead with no problems.
A smoothy with its lack of a rear sight has a limited range (60 yds for me) and that 60 cal ball will get it done.
I bump the load up in an effort to get a better blood trail, I like to blow completely through the deer.
Two leaks are better than one.
This doe met the 55gr load @ around 45 yds.
She ran about 50 yds and piled up.
I watched her drop - Between where I hit her and where she dropped there were 2 drops of blood.
Thats when I upped the charge.
Hope this helps