This is a good point & I have heard that from many folks. This aberration of constraints is puzzling, since most rules do have strict definitions & guidelines. I have read several rule books & the best I get is this ;
There are two types of racks, the commonly used triangle rack and the diamond rack, depending on the game. In any game of billiards, it is important to start off with a tight rack, meaning to set the balls up with no gap between the balls. Racks must always be set on the table with the point or apex directly over the foot spot of the table (located at the middle of the small side of the table in line with the second diamond on the long side). The Billiard Congress of America regulates the rules for professional billiards, including the racking requirements. Variations to these rules may be applied in amateur billiards, provided that the rack still conforms to the stated requirements.
This leeway can actually assist the breaker to make the wing ball consistently, by moving rack to one side of the spot.....
However, your opinion of center & my opinion of center may not agree. That may be the reason for the lack of precision constraints...
There are two types of racks, the commonly used triangle rack and the diamond rack, depending on the game. In any game of billiards, it is important to start off with a tight rack, meaning to set the balls up with no gap between the balls. Racks must always be set on the table with the point or apex directly over the foot spot of the table (located at the middle of the small side of the table in line with the second diamond on the long side). The Billiard Congress of America regulates the rules for professional billiards, including the racking requirements. Variations to these rules may be applied in amateur billiards, provided that the rack still conforms to the stated requirements.
This leeway can actually assist the breaker to make the wing ball consistently, by moving rack to one side of the spot.....
However, your opinion of center & my opinion of center may not agree. That may be the reason for the lack of precision constraints...