Start small, about 3/32" or smaller, as you can always enlarge the hole if you don't get enough vacuum. It's tough to drill it smaller! The "ported" vacuum should come in just above idle, so drill the hole to be exposed as the throttle plate opens a little. Vacuum on the distributor at idle makes it difficult to slow the engine down enough for a slow, smooth idle.
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!