Hi Garth,
I would always keep in mind that using specs to try and design a mattress without years of experience and knowledge can lead to some very unexpected results. The guidelines on the site are only generic and meant to help the members here understand the basic concepts behind mattress design but are not specific to any particular person.
There are far too many variables and unknowns to use specs as a way to choose a mattress (at best they can make a good starting point) and as you can see in mattress firmness/comfort levels in post #2 here I would either use your own personal testing or the guidance and experience of a retailer or manufacturer that can help you choose the design they offer that has the highest chance of success based on the “averages” of their customers.
Before you consider designing and building your own mattress out of separate components I would read post #15 here and post #2 here so you have realistic expectations and are comfortable with the higher risk and possibly higher costs involved. It can be a great project if what you learn and the challenge of building your own mattress is just as important to you as the final costs involved.
There are some good sources for individual components and zip covers in post #4 here but If you want a fire retardant barrier then it would need to be included in the cover. I would keep in mind that if the fire regulations are important to you that a mattress is always tested and burned as a single unit and not as individual components and some covers may pass the fire regulations with some designs and materials and not with others. Either way your mattress wouldn’t have been tested as a prototype at all.
Phoenix