I shoot 55 gr. Nosler BT varmint's. I called Nosler and they told me it will stabilize in a 1-14" twist (I shoot a Weatherby Vanguard.)
I would think with a 1-12' twist you could go slightly heavier, maybe a 60 gr..
But of course the length of the bullet also matters here, not just the weight..
The Greenhill formula says that your maximum weight and length for your caliber and twist rate is:
58 Grains and 0.63" in length, but this is just an estimate.
By the way, the specs on the Noslers I shoot are;
55 gr.
.0810 total length.
BC 0.267
These bullets are VERY accurate in my Vanguard..