Don't try to do it freehand. There are fixtures for the oldtime valve grinding machines that allow you to put a consistent radius on all the rockers. If you can't find a local machine shop that can do it, some of us old geezers on the bolt still have that equipment, and the US Mail still works- - - -sort of!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!