If you add
preserveAspectRatio="xMidYMin meet"
to the attributes of your svg element, that will keep the image at the top of the SVG box.
The obvious way to do what you want would be to use the preserveAspectRatio attribute, set the svg width to 100%, and style it with a CSS2 style having max-height: 70%.
Unfortunately, nothing (browsers, ADE, etc) seems to honour the max-height CSS attribute.
By setting height to 70%, things almost work, except when the image size is less than 70% because of the thin page. You then get white space below the image.
I don't think it's possible yet to do what you want. It shows how far ebook specifications and implementations have to come before they're properly fit for designing good looking books.
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