Next to sleeping position, your body weight and shape will play the biggest role in how well each layer relieves pressure and supports you for proper spinal alignment. There are many differences in needs between different people with different weights and shapes ... so lets take a look at what they are.
Overall body weight:
In general terms, the heavier and larger you are, the thicker a comfort layer you will need for each sleeping position. Heavier weights will also perceive a firmer foam as softer because the perception of softness is greatly affected by how far you sink in. This is because heavier people are generally larger and need a deeper cradle to spread their weight over the surface of the mattress to relieve pressure. Someone who is an average weight side sleeper may need 3" of comfort layering while someone who is heavier may need 4" for example.
In addition to a thicker comfort layer ... heavier people generally need a comfort layer that is a little less soft than lighter people. This is because the perception of softness is normally affected by how deep someone sinks into a mattress and how well it distributes pressure so a heavier person will need a firmer upper layer to form a pressure relieving cradle of the same depth than a lighter person. This firmer upper layer will likely feel as soft to them as a softer layer does to a lighter person.
In addition to this, a heavier person will need firmer support layers to prevent their heavier parts from sinking down too far and putting their spine out of alignment since their tendency will be to sink down into a mattress more than a lighter person.
Average weight = average thickness and softness of the comfort layers and average support layers.
Above average weight = thicker and firmer comfort layers and firmer support layers
Below average weight = thinner and softer comfort layers and possibly softer or average support layers.
Body shape and weight distribution:
Body shape and how your weight is distributed is just as important as overall body weight itself in choosing the perfect mattress. The curvier you are in your normal sleeping positions, the deeper a cradle you will need for pressure relief. In curvier sleeping positions and with curvier body profiles, there are more "gaps" to fill in to spread your weight over the mattress which means you will need a softer and thicker comfort layer than someone who sleeps in a flatter position and who does not have as many curves. On the other hand someone who has a "flatter" body profile and sleeps in a "flatter" position would be better off choosing a slightly firmer and thinner comfort layer. So too those who have "wider areas" (typically hips in women and shoulders in men) will need enough thickness and softness to accommodate these wider areas and relieve pressure. If you need a thicker comfort layer than normal, it should always go with a firmer support layer as well to "balance" the sinking down with the "sinking in".
The general rule here is that you deal with bigger gaps and curvier profiles with the thickness of the comfort layer.
You deal with weight by increasing the firmness of the comfort layers and support layers.
Average weight distribution and average body shape = average thickness and softness comfort layers and average firmness support layers
Uneven weight distribution and curvier than average body shape= softer/thicker comfort layers and firmer support layers
Even weight distribution and flatter than average body shape = thinner/firmer comfort layers and average support layers
So to recap since this is such an important part of your perfect mattress ...
Heavier weights will need firmer and thicker comfort layers
Lighter weights will need softer and thinner comfort layers
Curvier body profiles will need thicker and softer comfort layers
Flatter profiles will need thinner and firmer comfort layers
Uneven weight distribution will need firmer support layers
Even weight distribution will need average support layers.
Now we have dealt with your "statistics" we can continue on the journey by talking about your different preferences ...