You put the mirror flat on the scale. On high end analog meters, there was actually a mirror strip on the scale next to where the scale markings were printed.
To read a mirror scale, you wait for the needle position to stabilise, and then move your eye, so that the needle exactly covers the reflection. Then you read the scale where the need appears to point.
When the needle's reflection is completely obscured by the needle, you are looking at the scale precisely perpendicular, and there is no parallax error.
