Measuring Volume within Audacity
It is helpful to know whether your volume is correct before exporting to MP3. There is a plug-in for Audacity called ReplayGain which allows you to do this.
The latest version of ReplayGain.ny both analyses and has the facility for Equal Loudness Normalization (i.e. amplifying to the required level), and appears in the Effect menu in versions of Audacity 2.1.0 and later.
There is an earlier version of ReplayGain.ny which analyses the volume only and indicates by how much you need to amplify + (or de-amplify -) to achieve the desired volume of 89 dB. This version appears in the Analyze menu in 2.0.3 and in the Effect menu in 2.1.0.
The author of this plug-in (these plug-ins) explains the use of the plug-in in the Audacity forum here.