I can't speak from experience on tuning plates, as I've never personally used them. I considered them for my Klose, but decided against them, and haven't personally ever felt I needed them for my purposes. I think where they come in very useful is when you are cooking a large quantity of food (particularly on a mobile or other large pit) and absolutely want to be sure you can utilize as much space as possible with as much consistent heat as possible.
Regarding the Heat Baffle:
The baffle doesn't keep flame out. It is intended to deflect heat (and smoke) from immediately rising as soon as it exits the firebox. The firebox is set lower than the horizontal chamber.
I have a 20" diameter by 52" length horizontal, a 20"x20" upright, and a 20"x24" firebox:
The aperture from the firebox to the cooking chamber is only 15 inches wide where the firebox connects to the cooking chamber. It is semi-circular at the bottom to match the cylindrical bottom of the cooking chamber which it is flush with. The baffle is half-pie shaped just like the opening and is 15 inches long but only 5.5 inches at the widest point of the semi-circle. It may even be the very piece they cut from the side of the firebox and it appears to be angled down into the cooking chamber at about a 45 degree angle.
There is a "lip" in the horizontal chamber (you can see it rising from the bottom of the chamber, where the bottom of the yellow circle I made around the baffle in this picture runs across the lip:
I believe the lip is about an inch and a half, maybe 2" at the most. It is specifically intended to prevent grease from flowing from the horizontal down into the firebox and sparking a grease fire. If your pit is angled so the firebox is lower, it will not take much grease to overcome such a lip because it will build up at that end. So ensure that where you set your pit that you have a slight tilt to the opposite end.
There is an additional baffle from the horizontal into the upright:
You can see a nut at the top of the picture. While the baffle from the firebox to the horizontal was a welded crescent piece, this one is removable. While in place, the upright is about 75 degrees cooler than the horizontal. I've never bothered removing it because I use my upright for lower-temp uses, but removed it's supposed to bring the temperatures closer between the upright and horizontal.
Tuning plates are intended to allow normalizing temperatures across the breadth of the pit, assuming the pit is designed well to begin with. Specifically, tuning plates probably won't benefit a thin-steel pit (less than 1/4" thick steel) as much. Nor would they probably be as useful in a short horizontal chamber of under, say, 40 inches. In a 60" length pit as you mention, they would probably be helpful.
In the case of my 52" smoking chamber, the first 10-12" of the chamber closest to the firebox is probably 50-75 degrees hotter. The middle 30 inches is pretty consistent. The last 10-12" is a little cooler. There is also a heat differential between the upper and lower cooking racks.
As I understand it, the initial tuning plate is usually butted up to the heat baffle to immediately begin diverting heat underneath the plates. Each successive plate is gapped just a little wider from the one before it. There is a certain amount of radiant heat from the tuning plates themselves, depending on how close they are to the bottom rack (probably not as big an issue on your 24" diameter pit).
I'm not certain how most people mark the location of their tuning plates once they're satisfied with the location. I know some had numbered the plates and marked the locations with small weld marks.
However, a tuned pit will respond differently empty than loaded with meat, and even depending on the type and amount of meat loaded onto it. However, it will still provide less variation in temps across the pit than one without tuning plates.