Hi suffolk,
Oops … I hit submit on my last reply before it was complete.
This is very true and softness and firmness is completely subjective and varies with each person. One person’s “too soft” is another person’s “too firm” and there is no “formula” that can predict what someone will feel on a mattress outside of their own personal experience. Again though … I would always give a new mattress some time to get used to a new sleeping surface because your perceptions can change significantly over the course of the first few weeks.
In general … adding a topper to a mattress that is too soft can make an existing alignment issue worse because you would be adding even more soft foam to the top of the mattress which would “allow” your pelvis to sink down even further. firmer toppers will still “bend” into the softer layers below them.
Post #4 here has some suggestions that may be helpful.
Phoenix