Mountain Molds
Greenhill's Formula

If the rifling twist is one turn in  
And the bullet diameter is  
 
  the maximum bullet length  

This uses a constant of 150.   It does not predict the instability that sometimes occurs with blunt, short revolver bullets.

The formula is:    maximum bullet length   =   diameter   x   diameter   x   150   /   twist

The online design program's suggested twist uses a safety factor of 1.3.   This gives a good ballpark number but it is not intended to be the final word so don't lose a lot of sleep worrying about it.