Also try the Fade by dB plug-in ( https://forum.audacityteam.org/t/simple-plug-in-for-creating-partial-fades/16055/1 ) .
If you want % units you will have to try Text Envelope:
http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Text_Envelope .
Gale