Thanks,
I am new on Inkscape, for I am fed with fonts in SVG and in ePub (the combination I mean).
I need the borders, they are part of the design, they are not those of Inkscape but mine.
When I drew the SVG by hand and pocket calculator
in Calibre, I left blank/white spaces for all margins, but the SVG imported in Inkscape lost them and I have to use the above trick to get back to the good size.
I enclosed now the bare SVG in yet another object, a div, height:90%, margin-top: 5%, width:90%, margin-left:5% - this and the scaling to 100% within the SVG centered the cover.