I went with a 1-12 twist for my dedicated varmint rifle, but used a teke-off 1-8 Colt match barrel for my coyote gun. I knew I wouldn't be shooting anything heavier than 55 grains out of the p-dog rifle, so there was no need to over spin the bullet and risk instability. Yes, over spin can cause instability, but usually not keyholing like under spin. The 1-8 barrel was just what I could get, but I know I can go to anything that will mag feed and still get it stabilized.
Bullet velocity and barrel twist rate are the two things you need to calculate bullet RPM and therefore, stabilization. You can use a slower twist barrel if you push the bullet fast enough, and get the same bullet rpm as a faster twist barrel and a slower velocity.
In a 1-8 16" barrel pushing a 75 grain bullet at 2600 fps, for instance, the bullet rpm is 234,000. A 1-12 24" barrel pushing a 55 grain bullet at 3200 fps results in a bullet rpm of 192,000. A difference of only 18%. Substantially less rpm loss than the 50% reduction in twist rate alone would indicate. This is why some 1-9 twist 16" barrels won't stabilize a 75 grain bullet, while some 20" with the same twist will. The extra velocity from the longer barrel is just enough to spin the bullet fast enough to stabilize.
As far as single loading bullets, the only ones I know of that are too long to load in a mag are the 80 grain and up match stuff. Everything else, including the 75's and 77's should be loaded to mag length.
Fast Ed