Try mid-fade adjust at .707...Been working with this some doing up-mixes; and getting the correct phantom center fade ins & outs is tricky. Using "Adjustable Fade" in Audacity, and it takes a lot of trial and error to get it sounding smooth. Mid-fade Adjust (%) for both up and down is critical. Still trying to get a handle on it for 5.1, and of course every situation is different.
This has a dialog box where you can choose the shape of the fade in or fade out to be applied. You can also create "partial" fades to and from other than silence and original volume. An example of this might be a fade in from 20% of the original volume to 80% of the original volume. The "Handy Presets" at the bottom offer a choice of six pairs of fade in or fade out shapes, fading between silence and original volume.
Accessed by: Effect > Adjustable Fade...
View attachment 78777