If there is a general rule of " thumb " for determine the best Rate of Twist for round balls, it would be 1.5 times caliber:
.32= 1:48
.36= 1:54
.40= 1:60
.45= 1:67.5
.50= 1:75
.54= 1:81
.58= 1:87
.62= 1:93
.69= 1:103.5
.72= 1:108
These are the optimum ROT for these caliber RBs. You cannot overspin a Round ball. However, using a ROT that is faster than optimum tends to affect the size of variation in powder charge used that will still send the ball to the same POI.
For instance, My brother has several .40 caliber rifles that have a 1:48 ROT, and they shoot just fine. But even that ROT is much slower than the 1:16 ROT used in his friend's conical bullet .40 caliber rifle. He can still shoot RB in the gun but the powder charge is about 35 grains, instead of the 50 grain charge used for accuracy in the other .40 caliber rifle barrels made to shoot RBs with the 1:48 ROT.
The slower ROTs(shown) are much more forgiving to small variations in powder charges- as occurs when you use a volume measure to measure the powder, vs. using a powder scale to WEIGH the powder charge. The faster the ROT, the more likely you are going to need to both reduce the powder charge, AND weigh the charges to get small groups.
This is the reason we don't recommend buying a fast twist barrel designed to shoot conicals and sabots. Its not that you can't shoot RB out of these barrels: instead, its just more difficult to get good groups out of such barrels with RBs.
Now, KEEP in MIND that when we are describing group performance, we are usually talking about shooting a rifle off a bench rest, not off-hand. And, we are talking about shooting at distances of 50 yards and further. Many BP shooters never shoot at distances further than 50 yards, and sometimes not even that far, either in practice, or when hunting, since they are shooting off-hand mostly.