Thanks, DaleDe. The reference pixel was a new one for me. After a little cogitation, I see that it makes some sense – border lines etc. that are commonly given width in pixels will show uniformly across devices. And now I can have some confidence that all 6" e-ink kindles will have 600x800 reference pixels.
I'll check if the Fire also deals with reference pixels ASAP.
I still don't get why the eink kindles cannot deal with percentage dimensions in a sane manner, though.
Somewhat related, in
this book (very soon to be updated with corrections), I struggled with full-height, floating images, admittedly with ragged edges. Depending on font size, the first line on the next page was completely or partly cut off vertically. The workaround I've used is to reduce the vertical height to 96% (or thereabouts), and add a blank image, full-width and height 40px or thereabouts to ensure that if anything is cut off, it'll be the blank strip.