The way I'm doing it is somewhat akin to how in e.g. Android you design for various different screen sizes: I'm aligning everything to a grid, and am working with percentage offsets.
What I miss from SVG are anchor points and the ability to calculate offsets from those. I haven't seen much that would suggest it's possible so far.
I *could* go about all of this differently, of course, but there is also a point to this that involves understanding SVG better.