These are some preliminary thoughts as I try to understand layers.
Layering at 180 degrees is pulling the hair up from the bottom and cutting it flat to the ceiling. On top of the head this is the same as a 90 degree layering because the top of the head is nearly parallel with the ceiling.
The longest layers are 180 degree layers because the hair is pulled from hanging straight down to the flat plane above the head. On the sides the hair is pulled from 0 degrees as it hangs straight down to 180 degrees as it is pulled straight up toward the ceiling.
The term "longest layers" indicates that the length from where the layering starts to where it ends. Long layers end lower than shorter layers. Longer layers have more length removed from the higher part of the layering and less from the the bottom of the layering and this difference is spread over a greater distance.
On top of the head the longest layers are 90 degrees. There are no 180 degrees layers on top of the head.
Any layering less than 180 degrees is shorter than the 180 degree layering.
The shortest layers can be are when the hair is pulled 90 degrees from the head. Pulling less than 90 degrees is no longer layering and is instead graduation if the hair is cut perpendicular to the hair strands. If it is cut parallel to the wall (or straight up and down) the layering will have shorter lengths on top of the side of the head and longer layer sections in the bottom half of the head.
If you have a hair design with the longest layering on the top, you pull the hair straight up to 90 degrees.
If this design then calls for long layers on the side, continue to pull the hair on the sides up to 180 degrees and cut to the plane parallel to the ceiling.
If the design is for shorter layers on the side. Pull the hair on the sides to 90 degrees from the head form to the side and cut parallel to the wall.
For this kind of cut use the length on the top layers as the guide length for the layers on the side so they will connect smoothly.
Of course, on the side, layers can be any intermediate value from 90 degrees to 180 degrees depending on the desired effect or design.
Pulling sections to 90 degrees and cutting perpendicular to the hair shafts will give a uniform length at all places. Cutting at 90 degrees like this follows the head shape.
Flat layering can be done by pulling a vertical section to 90 degrees and cutting straight up and down. This is the same as cutting on a perpendicular line from the floor to the ceiling. This does not follow the head shape. Cutting straight up and down leaves the hair longer where the head shape curves away from the up and down line. It leaves the hair shorter where the head shape curves toward the up and down line.
These sections do not have to be cut straight up and down. For example, it could be cut shorter on the top of the section and longer at the bottom of the section. Here the shears are no longer on a perpendicular line from ceiling to floor. Now the shears are closer at the top of the head and farther from the head at the bottom of the section. This cutting does not follow the head shape. It follows a line that starts closer to the head and the top and slants away from the head as it goes down. The length of the hair increases both as the head shape curves away from this line and as the slant of the cutting line goes away from the head.
Cutting on this slanting line creates a concave section. This means that the length of the hair closest to the top of the head is shorter than the hair closer to the perimeter which is longer.
This type of cut would be used to cover a less dense hair line. In this case, the longer length of the hair near the perimeter will fall over the less dense hair line.
Cutting the hair with a concave shape can be modified to emphasize or to cover different parts of the head shape.
To build weight above the parietal ridge, cut the top short enough that the top of the section does not fall over the parietal ridge. This will leave weight above the parietal ridge which will diminish over the parietal ridge. The increasing length below the parietal ridge will add length and weight below the parietal ridge.
If weight is not needed above the parietal ridge, a longer length can be can be cut at the top that will fall over the parietal ridge. Below the parietal ridge the longer length will be continued by the concave cutting . This will add more length and weight below the parietal ridge to smooth exaggerated differences in head width.