If you use the pixel density 4 px/mm, 1 mm is represented by 4 px.
This line length is 100 mm, and this font size is 4 mm.
This svg has: width="150mm" height="70mm" viewbox="0 0 600 280".
Use only the unit millimeter (mm) in your svg image!
The html page code has:
<img src="testmm.svg" width="600" height="280" alt="SVG image"/>