I use it in my 308 and I've never needed to enlarged the flash hole. I have some paperwork somewhere that says to take the bullet your going to shoot and load it into an empty case at your OAL. Take a second bullet and set it next to the loaded round and mark the case where the base of the bullet sets. Pull the bullet and fill the case to the mark. Pour the powder into a scale. This is your max charge. It does not seem very scientific.
http://www.hodgdon.com/PDF/Trail-Boss-data.pdf
I shoot 11.6 gr of Trail Boss and a 225 Outlaw state bullet with an OAL of 2.740. The gun is a 20" Savage V=1055