Hi Djehuty,
The only way that anyone can be “certain” that a mattress is suitable for them in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is through their own experience in either testing a mattress or in their own sleeping experience. Beyond this you would be dependent on more detailed conversations on the phone with online manufacturers and retailers where you could provide them with enough information about your testing and your body type, sleeping positions, and preferences that they could help you make a choice of which of their mattresses have the best odds of working well for you. In this case the ability to re-arrange or exchange layers or any exchange or return policies may become more important in case you make a choice that is less than ideal in spite of your own “best efforts” and that of the manufacturer or retailer you are dealing with. There is more about this in post #2 here.
It’s certainly possible for a good manufacturer to use your local testing experiences on similar mattresses as part of the information that they can take into account in your conversations with them (if you can probide them with the specifics) but unless you have tested a mattress locally that is exactly the same as one they offer then there is some “translation” and “best judgement” involved and you will be going by “averages” that you would still need to confirm with your own sleeping experiences. Averages can work well for most people who are in an “average” range but if you are one of the minority who are outside the averages then you would be dependent on the options you have after a purchase to fine tune the mattress or exchange or return it.
Phoenix