Hi smmimp14,
There isn’t any way to “quantify” softness/firmness or how much of an effect the additional layer would have. Different people can have very different perceptions of firmness and softness compared to others 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 or layering combinations compare in terms of firmness/softness and different people may have very different opinions about “how much” softer an additional layer in a specific position in the mattress would feel. This is all relative and very subjective and is as much an art as a science.
About the only thing that I can say is that an additional medium 2" - 3" Dunlop layer would probably be noticeably softer than the same combination without the additional layer although the effect will be less if you are using it deeper in the mattress. If I was in your shoes and since your comments indicate that your current layering is only “a bit” too firm I would probably lean towards a 2" layer and then try it as either the second or third layer down to see which one is best for you. You can always add an additional inch of latex if you need to but you can’t take an inch away unless you have access to a slitter.
Experimentation is part of the DIY process (especially if you don’t have a specific layering combination that you have tested and are trying to approximate) so there isn’t any way to avoid this but I would also do some local testing on different latex mattresses or layering combinations so that you have a better reference point that you can use as a blueprint for your DIY design because you are the only one that can feel what you feel on any specific combination and “theory at a distance” is never as reliable as your own personal experience.
I would also make sure that you have a good exchange/return policy for any additional layer so that you have the option to exchange it for something firmer or softer if you need to.
Phoenix