A 1-in-12" twist won't fully stabilize more than about 52 grain lead-core bullets. 1-in-12 was designed for the idea that a .223 was a varmint round, using light bullets, from 35 to 52 grains or thereabouts. Once you move up to 55 grains, you're in the meta-stable region - neither unstable, nor stable enough to withstand all changes in shooting conditions (atmospheric density, wind, and loss of bullet velocity downrange).

Here's a good stability calculator. In the old days, when most all bullets were lead or jacketed lead cores, Greenhill's formula worked pretty well for twist rate. Today, with many new bullet designs, and all-copper or bronze match, long-range bullets, there's a little bit more involved in computing the twist required:

https://jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

The one parameter that is needed for twist that most people won't know off the top of their heads is the bullet length, a table of the more common bullets can be found here:

https://www.jbmballistics.com/ballistics/lengths/lengths.shtml

Here is the math behind the new bullet spin stability computations:

https://jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf