Bullet lenth and volocity give you the optum twist rate,if you are rebareling first choose a bullet lenth you want to youse,then the volicity you can achive( for instance a .971 long bullet traveling at 1350 fts the optum twist rate for stability is around 1-34 ,drop the velocity to 1200fts and twist drops to 1-32.7)this thory works realy well for BP cartrige rifles and was yoused by target shooters in the 1880s,it is not the Greenhill fomular,this is working of a .512 groove diameter and original 450gr style bullet