I also regularly need to export from Affinity Designer to SVG with correct dimensions. In my case I then convert it to DXF for CAD via Inkscape.
The short answer: Use the "SVG (digital - small size)" preset when exporting.
The only important thing is to export with 72 DPI (see "More" button). Just for reference: My tests showed that raster DPI, viewBox, relative coordinates or the document DPI and units are all irrelevant for accurate dimensions in the resulting SVG.
Another thing: When you use the trick of adding a surrounding rectangle of known size and then resizing everything in inkscape to that known size, reduce the line width of the bounding box to 0px.
Last note: I also use "Export text as curves" when exporting for CAD purposes.
