This article focuses on acoustic guitar bridge alignment.
There are at least two critical factors pursuant to correct alignment:
Occasionally, OEM applied bridges are coming off for one reason or another. Or, the original was shaved in an attempt to lower the saddle thereby lowering action. Or, intonation is just plain bad. 50 years ago, some companies did not pay close attention to intonation accuracy.
The most aggravating problem is too high action. Fifty years of string tension or high heat in a parked car causes this problem.
Now I realize a lot of people just do not have the funds available to pay for neck reset. And so shaving a bridge seems like a reasonable alternative to an expensive neck reset. But I can tell you that shaving the bridge usually ruins the bridge because it weakens the bridge to the point of cracking across the pin holes or in front of the saddle slot. This may well also result in a top cracked across the pin holes. And that is serious damage.
So my advice to you is never have the bridge shaved. Pay for the neck reset.
But I digress.
When replacing a pin bridge, the center line of the fret board must line up with the center located between the G and D string pin holes.
And the pin holes in the new bridge should line up exactly with the OEM pin holes. This assumes the hole were aligned correctly in the first place.
Also, there must be sufficient wood in front of the saddle slot to prevent or avoid breaking the bridge in front of the slot.