The Calibre Editor (and many others) is just a tool like a saw and hammer.
You need to know how to measure, cut and what the local building codes are.
Amazon has their building codes. EPUB has theirs. Some overlap.
Some you don't use because you want to publish for both and KISS takes front seat
over optimum.
In this case: Workshop is where you really want to be. Calibre and Sigil... Are just tools. You need advice on how to
build, not use the tool.