Hi Swisssleep,
Welcome to the Mattress Forum! all the way from Switzerland 
I would keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
I would also keep in mind that while other people’s comments about the knowledge and service of a particular business can certainly be very helpful, you are the only one that can feel what you feel on a mattress and I would be cautious about about using anyone else’s suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own personal preferences) may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
Hopefully some Hastens owners will see your post and share their comments and feedback but when you ask questions that are so subjective you may be putting yourself in a difficult or confusing situation because the replies you receive may be all over the map and there may be some people that tell you “I chose too firm” and others that tell you “I chose too soft” and others yet that tell you “I chose just right” and there will be no way for you to know which of the replies would apply to you.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you.
Based on your comments and feedback I would tend to agree with your salesperson. When you are looking at the quality of your sleep the most important part of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is your posture and alignment and to make sure that your spine and joints are in good alignment in all your sleeping positions. This is the single biggest factor that will affect the quality of your sleep and the ability of your body to rest and recover over the course of the night and how you feel when you wake up in the morning. A mattress that is too firm for your specific needs may not provide good support for the more recessed parts of your body (the lower back or waist) and can lead to sleeping out of alignment … just like a mattress that is too soft.
The next most important part of PPP (after alignment) is to make sure that your comfort layers are “just enough” in terms of thickness and softness so that they will relieve your pressure points in all your sleeping positions.
Posture and alignment and pressure relief are the two most important functions of any mattress and everything after this (sleeping temperature, ease of movement, motion transfer, edge support, etc) is more of a preference than a “need”. I would also keep in mind that you can only “feel” a mattress when you are awake and that posture and alignment and pressure relief when you are sleeping is always more important than the more subjective “feel” of a mattress.
Having said that … nobody on a forum can see you lying on the mattress and you are the only one that can feel what you feel on a mattress so using a “formula” based on specs (either yours or a mattress) or theory at a distance" that is only based on “averages” may not apply to your own specific needs and preferences and in most cases won’t be nearly as reliable as your own careful testing (hopefully using the testing guidelines in the tutorial along with the guidance of a knowledgeable and experienced retailer) to help you decide which model or firmness level would have the best chance of success.
Hastens certainly makes some exceptionally high quality hand built and tufted mattresses that uses high quality materials and components (pocket coils and natural fibers) but as you probably know they are in a much more premium budget range than many other mattresses that also use high quality and natural materials. There is more about Hastens and other “ultra premium” mattresses in post #2 here and post #2 here and post #2 here may also be of interest as well (the comments about Vi Spring would also apply to other ultra premium mattresses such as Hastens) but I would be very careful to differentiate how you feel “about” a Hastens and how you feel “on” a Hastens mattress.
There are certainly cases where a mattress in this budget range may be “worth it” for a particular person that isn’t price sensitive and that has specific criteria that aren’t available in lower budget ranges but this would be unusual and in general I would need a compelling reason that clearly indicated there was “enough” of a difference in “real life” compared to many other mattresses that may be just as suitable in terms of comfort, firmness, and PPP, just as durable, and that are in lower budget ranges to justify the higher cost.
Phoenix