Hi amr2625,
There is no way to quantify the specific layer that may be causing any issues or know how much each layer is compressing (outside of measurements in a lab) because all of the layers (memory foam and otherwise) interact and compress together and there are many different types of memory foam that each have different properties. This can only be done with personal testing or knowledge and experience with the mattress as a whole “system”. How much you sink into a mattress doesn’t matter as much as how “evenly” you sink in so I tend to use symptoms as a guideline along with the knowledge and experience of an actual retailer or manufacturer who would know more about how their specific mattresses compare than anyone else or any theory that didn’t take into account the specifics of their particular materials and designs.
The thickness and softness of a comfort layer along with the compression modulus of the material (how quickly it becomes firmer with deeper compression), it’s “point resilience” (contouring and conforming properties) and in the case of memory foam the environmental factors of temperature and humidity, the heat produced by each person, the bedding they use, their weight distribution, the temperature sensitivity and response time of the memory foam, and many other factors can all combine together to either produce good or poor alignment and different “feels”. It’s always a combination of factors in terms of how the layers interact together and with each person and the only way to put all the pieces of the puzzle together would be based on your own individual testing or experience or with the help and guidance of a knowledgeable and experienced salesperson (usually in a phone conversation where more nuanced information can be discussed much more easily and completely) who had your best interests in mind.
In general terms … thicker/softer comfort layers will “allow” the heavier parts of the body to sink “down” more before they “reach” the firmer support layers while at the same time they may “stop” the lighter parts of the body from sinking in enough if they are too firm. In addition to this, thicker / softer comfort layers can also provide better support to the parts of the spine that are more “recessed” and need to be in firmer contact with the mattress to help with good alignment. The specific properties of the comfort layers though will also modify and affect how much each part of the body sinks in or sinks down relative to the others because none of the layers act completely independently.
Thinner comfort layers will tend to feel firmer than the same materials in thicker layers because they will generally “allow” more of the feel of the firmer layers below them to come through. All else being equal they will also tend to “stop” the heavier parts of the body from sinking in more than thicker comfort laers. There are also many counter intuitive parts to mattress theory and design because in some cases softer comfort layers can feel firmer than a firmer comfort layer of the same thickness for some people because they may be sinking in more with the heavier parts of their body and feeling more of the firmness of the support layers underneath than they would if the same thickness of comfort layers was firmer. In other words … firmer can feel softer in some cases and thicker softer comfort layers can be more supportive (or more evenly supportive) in some cases.
One mattress that has a certain thickness of memory foam of a certain density may be very different from another mattress that used the same thickness and density of memory foam made by a different manufacturer on the same base layer because of the differences between different types of memory foam (see post #9 here and post #8 here).
More detailed generic information can be useful as a guideline or starting point or to help you know when someone is knowledgeable and experienced but as soon as you find yourself “studying” or trying to use more complex combinations of information as a way to choose a mattress you are most likely well on the way to “paralysis by analysis” or “information overwhelm” or making a choice that may be just as inappropriate as making a choice based on too little information.
You can quickly become overwhelmed with seemingly conflicting or overly complex information if you try to choose a mattress based on individual specs or use them as anything more than a generic guideline (outside of “quality” specs that can help identify weak links or the relative durability of two mattresses).
You will most often be much better off by “connecting with experts” who already have the knowledge and experience that you would otherwise need to take a great deal of time to learn than you would by analyzing individual specs and trying to imagine how they may feel or interact together without the experience and reference points that would make the more technical information meaningful.
The Regalis HD uses high quality materials and has no obvious weak links in the mattress so it would make a good choice for someone that was using the Rhapsody as a reference point. It would be more “supportive” under the heavier parts of your body. For those who wanted some additional softness (based on their personal experience from sleeping on the Regalis by itself) then the Regalis plus a topper could certainly make sense. Your own experience will be the real “test” of how either the Regalis by itself or the Regalis plus a topper will work for you and up until that point the “educated intuition and judgement” of the people you are working with along with your own testing and sleeping experiences and best judgement is the best way I know to make the choices that have the highest odds of success.
Phoenix