Hi,
in therory, the SINAD is independent of the vloume, because it is a relative measurement where the absolute audio level is cancelled.
In practice, you may have a residual hiss, when the volume ist set very low. If the volume is set verry high, you may have excess distortion.
The recommended audio level is normally specified in the documentation of the SINAD Meter. SINAD meters have a certain input range for accurate measurements.
If you know nothing, start with the volume control at 10/11 o´clock and use a 4 or 8 Ohm terminator for the audio line.
Regards
Chris