Hi Nemix,
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) as a reference point or blueprint and try and “match” every layer and component in your reference mattress (including any springs, any foam layers, and the cover) as closely as possible or use a “bottom up” approach (see post #2 here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
All the specs of all the layers and components of a mattress (including the cover) will affect the feel and performance of all the other layers both above and below it and the mattress as a whole but the thickness of any specific layer isn’t particularly meaningful by itself because it’s just one of many specs that will affect the feel and performance of a mattress in terms of PPP. There is more about the effect of thickness in post #14 here and there is also more about some of the many specs that can affect how a mattress feels and performs in post #2 here.
Unless you have a great deal of knowledge and experience with different types of mattress materials and specs and different layering combinations and how they combine together and can translate them into your own “real life” experience that can be unique to you (which would only be a very small percentage of people) … I would tend to avoid using individual specs such as layer thicknesses or ILD numbers or other complex combinations of information or specifications to try and predict how a mattress will feel or perform for you and focus more on your own actual testing and/or personal experience. When you try and choose (or build) a mattress based on complex combinations of specs that you don’t fully understand then the most common outcome is information overload and “paralysis by analysis”.
Phoenix