Re: RG570 - action higher at 12th than 24th fret
I've never used the 12th to measure action but sight the neck and set relief so the action is either even until the neck starts to bow, or decreasing, but never set by eye so that the action is ever increasing at any point from the last fret. Sighting the neck you can easily see what the fret line is doing right beside the straight edge of the string.
Otherwise buzz is a factor of physics, as long as it's even and not relieved at any single point where a high fret is causing the ones below to buzz. otherwise look at the ellipse of the string when plucked, it will be greater then your action at the points it's buzzing. Now barely pluck the string, less amplitude, no buzz. And your direction of pluck sets the primary path of the string vibration so picking perfectly parallel to the fret creates less buzz than picking down or deep toward the fret. At least to my ears which are long shot.