Hi jdg0928,
You can read a little more about trying to match one mattress to another in post #9 here. There is also more about primary support, secondary support, and pressure relief in post #4 here.
I would keep in mind that the top layers are primarily for comfort/pressure relief layers and the deeper layers are for primary support and the layers in the middle help with both to different degrees.
The Natura Twilight Eurotop is very different from your proposed Alexa design and it would be very difficult to translate because there are so many differences between them. They are completely different mattresses.
If I had to take a wild guess I would say that the Natura would feel a fair bit firmer because there is only 3" of latex in the top layers and then it more quickly transitions into the firmer (more supportive) 36 ILD latex underneath it. The Alexa would have the qulting foam on top which may be roughly equivalent to the 1" of 19 ILD in the Natura and the 3" of 28 ILD would make the comfort layers softer overall than the 2" of 28 ILD in the Natura (layer thickness and layer softness work together). The next layer down in your proposed Alexa design is also lower ILD (less supportive) than the support core in the Natura.
I would also keep in mind that how a mattress feels or how “supportive” it is when you sit on it with all your weight concentrated in one area has little relationship to how well it supports you when you are lying down and your weight is spread out over the length of your body. With such a thin layer of latex over the polyfoam how it felt it would also depend a great deal on the firmness of the polyfoam underneath it.
When you are trying to compare two mattresses that are so different then how it compares will also depend a lot on your body type and sleeping positions and how deeply each psrt of your body sinks in and it’s as much guesswork and imagination as anything else. I would keep your expectations fairl low of “matching” the Natura if that is your goal.
I don’t know anything about your partner’s body type but stomach sleepers generally do best with firmer or thinner comfort layers although this will also depend on their other sleeping positions and how long they spend on their stomach and in other positions.
It seems to me though that you may be mixing up the main functions of support layers (which provide the primary support) and the comfort layers (which provide the pressure relief and the secondary support that fills in the gaps in the sleeping profile).
I would also have a more extended conversation with Brooklyn Bedding because they will know more about their mattresses and how they might “match” another mattress than anyone but I would also think that they would be hesitant to guess when the Natura is so different. If anything I would tend towards one step firmer for you. Your sleeping position(s) will also make a difference.
Phoenix