I agree with 3-Hawks.
FWIW; here is my experience.
My own .69 cal requires a ball of .652, and a patch of .014......No and's, if's, or but's about it, that is what it has to be in order for my gun to shoot good.
Powder charges can vary a bit, but ball and patch simply has to be just right....a typical charge of Goex FFg in my gun is 120gr. I can punch paper with 60 / 70gr, but when it comes to serious shooting I like 110 / 120gr.
Everyone's mileage seems to vary on this, and there is no reason to think your gun will be any different....let the gun speak for itself!
I have been shooting the same .69cal since early 1970 and we have developed a very warm relationship.
Turner Kirkland would roll over in his grave if he knew the mileage I've gotten from one of his old $130 guns!
Remember, with a smooth bore, you have no lands / grooves for patch compression / displacement.
Something else to remember is the fact that don't want to have to "hammer" the ball in to get it started...if it is tight to start, it will be tight all the way down. You want a nice "snug to tight" fit from start to seating.
None of this can be discussed with any degree of accuracy until you know the actual diameter of your bore.
Once you know that, and remember the fact that a round ball has two sides, no matter how you look at it, then you will be able to get yourself a roundball mould, and determine the thickness of the patch you will be needing.
Also, there is a good chance you might want to consider Jeff Tanner, as the source of that mould, once you determine the size mould you will be needing....a "custom" mould for just a bit more than Lee prices.
Standard commercial moulds are sort of a "one size fits all" and although it's possible that your smoothie just may fall right in line with this, chances are it won't....so be prepared.
Smooth bores can shoot as well as any rifle, given the proper ball / patch fit...in fact, there is a good chance you are going to be pleasantly surprised after just a few trips to the range.
Perhaps the best advice anyone can give at this point is; don't expect a ill fitting ball to shoot well! It ain't going to happen, and it will give you a lot of unnecessary grief.
Get it in your hand, then let's talk.
Uncle Russ...