I don’t know if this will help, but you may find it a bit easier with this plug-in effect: https://forum.audacityteam.org/t/change-speed-stretch-audio-to-a-specified-length/23332/1
It is much like “Change Speed” but you can specify the desired duration rather than having to guess the percentage.
To install Nyquist plug-ins: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#Installing_Plug-ins