Good point, willshoum. Length actually is the more important factor and if this were an exact science we'd probably use that factor. And we'd have to bring in how an individual barrel actually works, not how its twist rate usually works.
But since we can generalize that longer bullets are also heavier and that it's easy to read the bullet weight right off the box where bullet length is something you have to dig harder for - weight is the factor most people discuss when twist rate comes up.
I don't know how long a 55gr or 69 gr bullet is and most people don't. So we tend to use this shortcut since it works well enough most of the time.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek