The live center should be tightened enough that it spins with the mandrel but not so tight that it bows the mandrel . If it is not tight enough the mandrel will place uneven pressure on the live center causing more wear on the point of the center and the socket of the mandrel which could also cause an OOR problem .
The way I do it is to start with the tailstock quill all the way in (retracted) , then I bring the tailstock up until the live center fully contacts the mandrel socket , then I lock the tailstock down . Then I start the lathe and tighten the quill until the live center is spinning with the mandrel then I lock the quill . You can hear when the live center is spinning at the same speed as the lathe .
Others may do it differently but this method has worked for me since I started turning and I have never bent a mandrel or wore out a live center .
